Chelsea also rallied to earn a 1-1 draw at Eintracht Frankfurt, with Pedro Rodriguez scoring the potentially crucial away goal in Germany to extend his team's unbeaten run in the competition to 16 games — a record.
Neither Arsenal nor Chelsea is guaranteed a top-four finish in the Premier League, which earns automatic qualification for next season's Champions League. Winning the Europa League also offers that reward, which is why the teams have taken the competition seriously.
The final is in Baku, Azerbaijan, on May 29. It had previously been a disappointing week in Europe for English clubs, with Tottenham and Liverpool losing the first legs of their Champions League semifinals against Ajax (1-0) and Barcelona (3-0), respectively.
MR. EUROPA LEAGUE A three-time winner of the competition with Sevilla, Unai Emery is on course for another Europa League final in his first season at Arsenal. He has his lethal strike partnership to thank for that.
The potency of Lacazette and Aubameyang continues to make up for Arsenal's weakness at the back, which was exposed again when Valencia took the lead off a corner in the 11th minute. A delivery to the far post was headed back across the face of goal by Rodrigo, and Mouctar Diakhaby rose above Arsenal midfielder Granit Xhaka to nod in from close range.
Arsenal has been saving its best performances for the Europa League of late — back-to-back wins over Napoli in the quarterfinals were followed by three straight losses in the Premier League — and the English team hit back almost immediately through Lacazette in the 18th.
He played in Aubameyang with a superb through-ball and was then on hand to receive a squared pass from his fellow striker to stroke the ball into an empty net from the edge of the area. Lacazette made it 2-1 eight minutes later when his header from Xhaka's first-time cross could only be palmed over the goal line by Valencia goalkeeper Neto.
Lacazette missed out on a hat trick by wasting two clear-cut chances in the second half, but Aubameyang gave Arsenal a two-goal cushion heading into the second leg next Thursday by volleying home at the far post from Sead Kolasinac's floated cross.
"I hope that third goal will be important," Aubameyang said. "We started a bit nervous because we lost the last three games, I think that's normal. We are human, but after that we had a good reaction."
LOFTUS-CHEEK IMPRESSES With Eden Hazard only starting on the bench, Chelsea needed another player to be its attacking inspiration and it proved to be Ruben Loftus-Cheek. The powerfully built England midfielder has had to bide his time this season, with Mateo Kovacic and Ross Barkley often starting ahead of him, but he has forced his way into the starting lineup in recent weeks and was Chelsea's best player in Frankfurt with his passing and runs from deep.
Fittingly, it was Loftus-Cheek who set up Pedro's 45th-minute goal, shrugging off a couple of defenders and laying the ball off to the Spanish winger, who shifted it onto his left foot and swept in a finish from 15 yards (meters).
That canceled out Frankfurt's opener from Serbia striker Luka Jovic, who glanced in Filip Kostic's cross for his ninth goal of this season's competition — a tally bettered only by Chelsea's Olivier Giroud.
Chelsea defender David Luiz struck a free kick against the crossbar in the second half but the visitors were hanging on near the end, with Sebastian Rode heading over from close range and Giroud getting away with a handball in his own area. The video assistant referee system is not being used in this season's Europa League until the final.
"I think, in the end, we deserved the win," said Chelsea manager Maurizio Sarri, who is looking to win his first trophy in 29 years as a coach. "But 1-1 away is not so bad."
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Steve Douglas is at www.twitter.com/sdouglas80