Last Updated: 10/04/13 9:41pm
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It may have been Pedro who scored the goal which took Barcelona through, but Messi was the architect and he changed the mentality of his side when he was introduced from the bench.
Nasty collision between van der Wiel and Jordi Alba leaves both men on the floor, but they are OK to continue.
Ibrahimovic heads goalwards, but cannot get any real power on his effort and Valdes gathers under his crossbar.
Great skill from Iniesta as he turns Jallet inside and out to get into the box, only to blaze his shot into the side-netting.
Thiago Silva goes into the book for bringing down Messi, another caution which will lead to a suspension.
Lavezzi goes into the book for clipping Jordi Alba. He'll miss the next game should PSG regain the lead.
The transformation in Barcelona with the introduction of Messi was simply staggering. One man should not make a team, but on this occasion....
Have regained control, Barcelona are now more happy to pass the ball around and wait for PSG to put them under perssure.
Messi picks the ball up in the PSG half before feeding Alves out on the right. The defender's deep cross is then just too high for his team-mate as he races into the box.
Brilliant, brilliant play from Barcelona, with Messi at the heart of it. He opens up the PSG defence to find David Villa, who rolls the ball back into the path of Pedro to rifle home.
Alves gets forward down the right and cuts inside, but is unable to pick a pass. PSG then break but the move comes to nothing.
More danger for Barcelona as Alex and Ibrahimovic play a one-two with Valdes alert and able to claim the ball.
Pastore is afforded far too much space in the Barca half and is allowed to advance towards goal before firing wide.
Iniesta brings a diving save out of Sirigu after a one-two with Villa and the rebound falls to Messi, but his first touch is unusually poor and the chance is gone.
Xavi's corner drops to Pique in the PSG box but his drilled low effort is blocked before it can trouble Sirigu.
We're about to find out just how fit Barcelona's talisman is - will he be the hero, or will this gamble backfire?
Ibrahimovic is body-checked by Pique on the edge of the Barcelona box but is then inexplicably told to get to his feet by the referee.
Ibrahimovic warms Valdes' palms with a free-kick from 30 yards which the goalkeeper always had covered.
CHANCE FOR PSG!! More good play from Ibrahimovic as his pass finds Pastore well placed in the box, but he makes a hash of the shot and it flies wide of the post.
David Villa's shot takes a huge deflection off Alex's knee, but the referee ignores their pleas for a corner and gives the goal-kick.
Jordi Alba looks to take on Jallet but cannot even earn a corner as the last touch comes off the Barca man.
This will be a real test for Barcelona now and it looks as though Messi - regardless of how fit he is - could be coming on soon.
Ibrahimovic is close to finding Lavezzi with another fine pass, but Barcelona clear the danger on this occasion.
Brilliant counter-attack from PSG. Pastore combines superbly with Ibrahimovic before breaking into the area and clipping the ball over the advancing Valdes and into the net.
Another long-range effort from Fabregas. Another shot which only troubles the crowd behind the goal.
Referee Kuipers brings the half to a close. PSG have had the better chances in an open first-half, but they are still behind on away goals.
Iniesta's pass somehow pinballs through to Fabregas, but Maxwell is alert to put it out for a corner.
Dani Alves has the chance to cross but over-hits it horribly and sees the ball drift out for a goalkick.
Ibrahimovic takes charge of the resulting free-kick but blazes his powerful effort straight into the Barcelona wall.
Fabregas is the latest player to try his luck from range, but just like so many before him he gets under the ball and it flies well over the bar.
Pastore's pass finds Ibrahimovic bursting into the area, but Valdes is able to gather, and the flag had already gone up for offside.
Xavi steps over Pedro's pass and lets it run to David Villa, who fires wildly over the bar, much to his frustration.
Jallet is down and needs some treatment after an accidental clash with Pedro, but is soon back on his feet.
Pedro takes on Jallet and gets to the byline before blazing the ball wildly across goal when he would have been better served attempting to pick a pass.
Lucas swings the ball into the area and it flicks off the top of Alex's head, with Ibrahimovic arguably better placed just behind him.
Pastore finds Lucas as he runs into the Barca area, with the home side conceding first one corner and then another.
Barca look to take some of the sting out of the game with a spell of passing, particularly with Alves off the pitch briefly changing his boots.
From the resulting corner, Alex meets the ball and heads down and towards goal with Valdes diving to his right to save again. The flag has, however, gone up for offside against Lavezzi.
GOOD SAVE!!! Valdes is called into action once again, pushing a strong far post header from Lucas up and over the bar after an excellent deep cross from Ibrahimovic.
Lavezzi is found in space on the left of the Barca box but his attempted pull-back is poor and the home side clear.
Good play from Lavezzi, working back in his own half to regain possession and then showing strength to break away before being unfairly felled by Pedro.
Close call for Barca as Valdes is called into action, getting down well to deny Lavezzi after PSG worked another opening through Ibrahimovic.
Quick feet from Iniesta as he receives the ball with his back to goal and surrounded by four defenders. He gets a shot away but it is blocked by Maxwell.
Wasteful from Pastore as he fails to pick out either Lavezzi or Ibrahimovic when well placed just inside the Barca half.
Pedro does well down the left and fires over a low cross which Sirigu pushes away. The ball comes back to the Barca man and his second attempt finds the side netting.
Jallett finds Lucas, who then attempts a one-two with Lavezzi, but the striker's return is too strong as his team-mate looks to burst in behind the Barcelona defence.
Another shooting chance for the visitors, with Ibrahimovic finding Lucas in space outside the box, but again his powerful shot is well off target.
Alves fires over a low cross which David Villa meets only to see his shot from 12 yards deflected behind for a corner.
Pedro and Iniesta continue to probe the PSG defence, but Maxwell heads the former's cross clear before Jallet does well to deny the midfield ace.
Now Lavezzi works a yard of space for a shot from outside the box but can only blaze high over the crossbar.
Valdes is called into action again, dropping to his right to claim a low shot from Lucas from the edge of the box.
Jordi Alba sends Lucas flying into the advertising hoardings behind the goal with a sneaky push as the midfielder looks to beat him, but only a goalkick is given.
Iniesta attempts an ambitious lob from 35 yards out which has Sirigu back-pedalling, but is always going over the crossbar.
Typically neat passing and movement from Barcelona before Xavi lokos to feed Fabregas, but his run is cut off.
Shocking pass from Pique as he gifts the ball to Ibrahimovic on the halfway line, but the big striker fails to take advantage.
Xavi's corner finds Pique, but his audacious attempt to back-heel the ball towards goal comes to nothing.
Iniesta and Fabregas exchange passes on the edge of the PSG box before the former drills in a shot which is blocked for a corner by Jallet.
Fabregas goes to ground as he shoots from the edge of the area under pressure from Thiago Motta, but the referee is not interested.
Thiago Silva slips as he attempts a free-kick and then boots the ball out of play, giving Barca another opportunity just outside the PSG box.
Lavezzi steals the ball from Busquets in midfield and bears down on the Barca goal, but Adriano and Pique get back before Valdes clears.
Iniesta looks to thread a pass through the PSG defence but Alex reads it well and cuts the danger out.
JUST WIDE!!! Xavi curls a superb free-kick up and over the wall which ripples the side-netting as it beats the diving Sirigu. Plenty of people in the ground thought that was in....
Just a quick reminder that Barcelona currently hold the advantage in the tie courtesy of the two away goals they scored in the first leg last week. The only result which will see us go to extra time and penalties is a repeat of that 2-2 scoreline.
Tonight's referee is Bjorn Kuipers from Holland. Fingers crossed he doesn't get too many mentions, as that generally means he'll have had a good game.
The Champions League anthem rings out around Camp Nou with kick-off now just a couple of minutes away.
And Aiden (or @haggisherder) reckons: "As a neutral I'm expecting a cracking game. A part of me wants PSG to scrape it simply because of the class of Ibrahimovic."
George Douglas writes: "I think PSG are gonna go all out and attack so I'm going with a 2-0 win to PSG."
Hasan Moosa, however, says: "I think Beckham played really well in the first leg so am surprised to see him on the bench. PSG will miss Matuidi, and Barcelona will have just too much quality at home. 4-1 Barca with Messi to score off the bench. However the scoreline will flatter Barca."
Will Hunter is another not in the Beckham-mania camp, as he writes: "By 'less convinced by his display' do you mean: not blinded by arrogant journalism to see that the game bypassed him? He has no pace any more and needs to have the ball to pass it, not going to happen often against Barcelona."
Thiago Silva and Ezequiel Lavezzi are the men walking a disciplinary tightrope for PSG, with first-leg goal hero Blaise Matuidi sitting this game out through suspension.
Jordi Alba, Gerard Pique and Adriano will have to tread carefully, with the Barcelona defensive trio facing the threat of missing the semi-final - should they progress - if they are booked tonight.
It would be fair to say that @evenewtonjobs is not a fan of David Beckham. He writes: "Want Barca to win badly maybe for no greater reason than i'm so sick & tired of hearing Beckham's name. Go away! Never has such an average over the hill player received so much unwarranted attention." Don't sit on the fence, tell us what you really think....
Paris St Germain are 7/1 to progress by winning tonight with Barca the strong favourites at 2/5, while another draw is 15/4. With Messi benched, David Villa is the 7/2 favourite to score first while Zlatan Ibrahimovic is price boosted to 17/2 to open the scoring against his former club.
Messi misses out on a place in the Barca starting XI after injuring his hamstring in the first leg in Paris last week. Beckham started that game for PSG and was lauded by the English press, although many in France were less convinced by his display.
Ali Raza has been in touch to say: "Tonight is going to be a cracker. PSG will look to break through Barca's defence through the physical strength of Ibra but it seems to me an easy win for Barca. Villa to score an early goal."
Rafee Ahmed says: "What a sensational match this is just can't wait for the match to start with both sides full of stars, now we will see the real face of Barca without Messi and I think PSG will win today from the big man Ibrahimovic and show Barcelona what they are missing... 2-0 to PSG."
Chris Green from Huddersfield writes: "I'm really looking forward to this match. It's going to be a cracker and hopefully, there will be plenty of goals for us to enjoy. Tonight's match will be tough for Paris St Germain as they are facing probably, the best in the world at this moment in time, but they have every chance of winning the game if they perform to their potential. For any team, coming to the Nou Camp is difficult because the atmosphere inside the stadium can affect their confidence, but I'm sure Paris St Germain will cope with it. It would be lovely if Paris St Germain caused a major shock and beat Barcelona, but I can't see this happening if Barcelona start the match positively. If Paris St Germain are to cause a shock here this evening, they must take their chances and stop Barcelona in their tracks; otherwise, Barcelona will win this match conformably. My prediction: Barcelona 3- 1 PSG."
If you want to get in touch and have your say on the game, then drop me a line to email@example.com or via Twitter @skysportsrobp and I'll publish a selection of the best prior to kick-off.
So Lionel Messi and David Beckham are only on the bench for their respective teams, but there is more than enough attacking talent on both sides to ensure tonight's contest will be an enthralling clash.
PSG starting XI: Sirigu, Jallet, Alex, Thiago Silva, Maxwell, Lucas, Verratti, Thiago Motta, Pastore, Ibrahimovic, Lavezzi. Subs: Douchez, Sakho, Gameiro, Chantome, Armand, Van der Wiel, Beckham.
Barcelona starting XI: Valdes, Jordi Alba, Adriano, Pique, Alves, Iniesta, Busquets, Xavi, Villa, Fabregas, Pedro. Subs: Pinto, Alexis Sanchez, Messi, Thiago Alcantara, Bartra, Montoya, Song.