Michael Obafemi and Nathan Redmond were on target as Southampton consigned Chelsea to a second consecutive defeat at Stamford Bridge.
Academy product Obafemi, starting in place of the rested Danny Ings, gave Southampton the lead when he curled home a stunning left-footed effort from the edge of the box (31) after Chelsea had lost possession in midfield.
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It was the 19-year-old's first Premier League goal in a year and things went from bad to worse for Chelsea when Redmond finished off another slick Southampton attack (73) in the second half.
Frank Lampard's men dominated possession but showed none of the cutting edge that helped them beat Tottenham last time out, allowing Southampton to claim a deserved win which lifts them up to 14th in the Premier League table.
Chelsea: Kepa (6), Azpilicueta (5), Rudiger (6), Zouma (6), Tomori (6), Emerson (5), Kante (6), Jorginho (6), Willian (6), Hudson-Odoi (5), Abraham (5)
Subs: Mount (7), Pulisic (6), Pedro (6)
Southampton: McCarthy (7), Soares (7), Stephens (9), Bednarek (8), Bertrand (7), Armstrong (8), Ward-Prowse (8), Hojbjerg (8), Redmond (8), Obafemi (8), Adams (7)
Subs: Ings (6), Romeu (6), Boufal (6)
Man of the match: Jack Stephens
How Saints stunned sluggish Chelsea
Chelsea came into the game in buoyant mood following that excellent win over Spurs, but they looked sluggish from the start and struggled to create meaningful chances, with Fikayo Tomori sending a tame header straight at Alex McCarthy with their only effort on target in the first half.
A string of promising attacks broke down due to poor crosses and a lack of composure in the final third, with the home fans showing their frustration in the stands, but Southampton also defended superbly, with Jan Bednarek and Jack Stephens marshalling Tammy Abraham closely.
- There were three Chelsea changes, with Emerson, Jorginho and Callum Hudson-Odoi coming in for Marcos Alonso, Mateo Kovacic and Mason Mount.
- Southampton made two changes, with Ralph Hasenhuttl electing to rest Danny Ings and drop Shane Long. Michael Obafemi and Che Adams started in their places.
The visitors were a threat on the counter-attack too, and it was from one such situation that they took the lead, springing forward after Callum Hudson-Odoi was dispossessed under the nose of the watching Lampard. Within a few seconds, Obafemi was celebrating a memorable goal.
Lampard threw on Mason Mount for Kurt Zouma at the break, changing his system from a back three to a back four, and the substitute made an immediate impact when he teed up Abraham, who fired a diagonal effort into the side-netting just seconds after the restart.
But Southampton defended resolutely and continued to look dangerous on the break, with Redmond's pace and directness causing Chelsea problems.
In fact, the winger could - and probably should - have doubled Southampton's lead before he did, running through on goal in the 66th minute only to be denied by Kepa Arrizabalaga's low save.
He made amends not long afterwards when he clipped a cool finish over the Chelsea goalkeeper after N'Golo Kante had inadvertently guided the ball into his path, and the hosts were unable to recover. There was a late free kick from Mount which forced McCarthy into a save, but by the end of the game, Chelsea had only managed three shots on target all afternoon.
The result makes it two consecutive home defeats against lower-half opposition for Lampard's side following the 1-0 loss to Bournemouth earlier this month, leaving Lampard to bemoan the "same old story" in his post-match press conference. Chelsea now sit just three points ahead of fifth-placed Tottenham in the table
Lampard: We need to do more offensively
Frank Lampard: "It's the same story as a few of the last home games. I said after Spurs that we need to make sure we reproduce that kind of performance, but I'll probably give you the exact same press conference as I did after Bournemouth to be fair, because it's the same story.
"Teams aren't stupid. They are well organised. We need to do more in an attacking sense. You can't have more than 70 per cent of the possession and not do more, not hit better crosses, not hit better final passes and not get more clear chances at goal.
"You need a form of consistency to finish in the top four because this is the Premier League and everyone is striving for it. We're in it at Christmas and people probably would have doubted that, so I think with perspective we can be relatively happy.
"But if we want to stay there and look onwards and upwards rather than backwards, we need to win more than we have won recently at home."
Hasenhuttl: Best performance of the season
Ralph Hasenhuttl: "You can have to have a tactical plan, but if you don't have passion, it doesn't work.
"I'm most proud about the moments when we won the ball because, especially in the second half, we showed that we can play some good passes and I think this is important against such an opponent because when you only defend, it comes to a moment when the pressure gets too much and you lose such games.
"It's something that has happened very often in the past but today it didn't because we stayed brave and we tried to keep them far away from our goal. In the end I think it was by far the best performance this season. We had a few very good away games, but I think this was the best one."
Opta stats: Chelsea's dismal December
- Chelsea have lost back to back home Premier League matches for the first time since November 2011 under Andre Villas-Boas.
- Chelsea have lost seven Premier League games this season - only one fewer than they lost in the entirety of the 2018/19 season under Maurizio Sarri.
- Southampton have won four of their last six Premier League matches, one more than in their previous 20 games.
- Chelsea had just 10 shots against Southampton, their fewest in a Premier League game under Frank Lampard and fewest since April against Liverpool (six).
Chelsea face Arsenal live on Sky Sports Premier League from 1pm on Sunday at the Emirates Stadium (kick-off 2pm), while Southampton are at home to Crystal Palace on Saturday, kicking off at 3pm at St Mary's Stadium.