Pierre-Emerick Aubameyang's late goal gave Arsenal a commanding 3-1 lead over Valencia after the first leg of their Europa League semi-final.
The Gabon striker volleyed in from close-range in the final minute to give Arsenal a more comfortable advantage than they had seemed likely to take back to the Mestalla next Thursday, turning in Sead Kolasinac's pinpoint cross.
Things had looked like panning out very differently with Valencia starting the brighter at the Emirates, and Mouctar Diakhaby headed in from a corner to cement their early dominance (11).
But Alexandre Lacazette single-handedly mounted the Gunners' first-half turnaround, first combining with Aubameyang, who he played through with a wonderful 40-yard slide-rule pass before taking the return in his stride and levelling with Arsenal's first shot of the game (18).
Arsenal: Cech (7), Maitland-Niles (6), Mustafi (8), Sokratis (7), Koscielny (6), Kolasinac (6), Guendouzi (6), Xhaka (8), Ozil (7), Aubameyang (9), Lacazette (9).
Subs: Mkhitaryan (6), Torreira (7), Monreal (n/a)
Valencia: Neto (5), Piccini (5), Garay (6), Gabriel (6), Roncaglia (5), Gaya (7), Soler (5), Parejo (8), Diakhaby (7), Guedes (6), Rodrigo (7).
Subs: Gameiro (7), Wass (6), Santi Mina (n/a).
Man of the match: Alexandre Lacazette
And as the hosts grew in confidence, he then headed in from Granit Xhaka's cross despite Neto's desperate attempt to keep his effort from crossing the line (26).
His double, added to Aubameyang's late third which was met with wild celebrations from Unai Emery, leaves Arsenal in pole position to reach their first European final since 2006 ahead of next week's second leg.
How Arsenal turned despair to glory
It looked an all-too familiar story for out-of-form Arsenal early on as Valencia took control early on, and they were given a major let-off when Daniel Parejo's free-kick was turned across goal and Ezequiel Garay volleyed over from a great position inside the six-yard box.
Any regrets he had were soon wiped out though, when some more charitable Arsenal defending contributed to the visitors' opener. Parejo's corner was met by Rodrigo at the far post and headed back across goal, where Diakhaby beat Granit Xhaka, Lacazette and Mustafi in the air to head beyond Petr Cech.
Valencia began to turn the screw at an already-frustrated Emirates but shortly after Goncalo Guedes turned an awful Ainsley Maitland-Niles backpass over the bar, Arsenal lit the spark that would ignite their performance.
Mesut Ozil fed Lacazette on the half-way line and after turning Cristiano Piccini with ease, he slid the ball through the eye of a needle to find Aubameyang on the break 40 yards ahead. He held off two defenders and drew Neto out of his goal while waiting for Lacazette's run, before squaring for his strike partner to fire into an empty net from 18 yards.
Live Renault Super Sunday
Suddenly this was a different Arsenal. Lacazette was given another glorious chance eight minutes later when Xhaka's cross from deep found him unmarked on the right. Neto made a mess of his near-post header, pushing it onto the post before clearing, but a buzz on the referee's watch indicated the ball had already crossed the line for Arsenal's second.
The hosts were in control of the match but questions remained of what this Valencia side had left in its locker. After half-time, any worries of a second-half backlash were gradually extinguished as the Gunners mounted a largely disciplined attempt to see the game out, while still looking for an all-important third if they could get it.
- Arsenal started Mesut Ozil and Matteo Guendouzi but dropped Lucas Torreira, with four changes in total
- After defeat to lowly Eibar last time out, Valencia made five changes
Lacazette should have been the man to deliver and had a glorious chance when Xhaka's shot deflected into his path, but from eight yards out on the left he let Neto keep Valencia in the game with two easy saves in quick succession.
That was nothing, however, compared to his miss after Aubameyang had tricked his way past Gabriel and teed him up at the far post, only for the Frenchman to waste a glorious chance for the match ball and miss the ball altogether with a diving header - and he was nearly made to pay when substitute Kevin Gameiro came close at the other end, only for Cech to deny him with a smart save.
Just as it looked like those misses would catch up with Arsenal, Kolasinac finally produced a quality delivery from wide when it mattered most, finding Aubameyang's run to the back post, with his volley putting a very different shine on an already impressive comeback from the Gunners - and leaving Valencia an uphill task at the Mestalla next week.
- Arsenal have progressed from 37 of their 40 ties in all European competition when winning the first leg, though Valencia were one of the three teams to eliminate them, in the 2000-01 Champions League quarter-final.
- This is the 28th time Valencia have lost the first leg of a two-legged European tie - they've ended up winning the tie on 11 previous occasions, most recently against Basel in the 2013-14 Europa League.
- Mouctar Diakhaby's opener for Valencia was the first home goal Arsenal had conceded in the Europa League since the group stages, ending a run of 460 minutes without conceding for the Gunners at the Emirates.
- Alexandre Lacazette became only the third player to score a brace in a European semi-final match for Arsenal, after Charlie George in the 1969-70 Fairs Cup and Steve Bould in the 1994-95 Cup Winners Cup.
- Arsenal's Pierre-Emerick Aubameyang has been directly involved in 30 goals in his 28 starts at the Emirates in all competitions (23 goals, 7 assists).
- 27 of Alexandre Lacazette's 35 goals in all competitions for Arsenal have come at the Emirates Stadium (77%).
The Debate: An all-English final?
Alan Smith: "It was a big goal from Aubameyang for Arsenal. At 2-1, you weren't sure but they have got that extra goal cushion. If they go to Valencia and performed how they did against Napoli in Italy, there won't be too many problems.
"From what we saw tonight, Valencia have got a bit more about them than Napoli did. At the start of the campaign, you thought Napoli were one of the strongest in the tournament.
"It is a big win for Arsenal, especially going a goal down fairly early on and looking unsure, it was a really poor goal to concede from the corner. But they recovered, they rallied and Lacazette again got a couple of goals, some important ones and it was another big goal from Aubameyang. He's got a track record of that this season."
Emma Hayes: "I think most people, looking at the balance of play, would argue that both English clubs are likely to make it [to the final], although it is never as straight forward as that. I think Arsenal's third goal cushion makes the difference for them but at 1-1 for Chelsea, perhaps there is a little bit more work to do.
Arsenal host Brighton at the Emirates live on Sky Sports Premier League and Sky Sports Main Event as part of Renault Super Sunday from 4.15pm; kick-off at 4.30pm.
Valencia, facing an uphill struggle to finish in the top four in La Liga, visit Huesca at 7.45pm on Sunday.
When is the Europa League final?
The Europa League final, the 10th in its current format and 48th overall of Europe's second-tier competition, will take place on May 29, 2019.
Kick-off is at 8pm (BST).