Plenty of nervous Arsenal supporters were packed inside the Emirates Stadium but they were quickly calmed down when Atleti right-back Sime Vrsaljko received two yellow cards in the opening 10 minutes of the match.
Already furious at the dismissal of Šime Vrsaljko after just nine minutes, the former Argentina global reacted angrily when an Arsenal player's mistimed challenge didn't receive a yellow card moments later.
Arsenal dominated the first half but couldn't make their advantage count as we wasted a number of great chances to break the deadlock and it was 0-0 at half time.
Wenger's path as Arsenal boss has not been smooth in over a decade and his side managed to all-but gift the visitors what could prove a pivotal equaliser.
A long ball forward was not dealt with by Koscielny, whose clearance hit Griezmann with the striker's first shot squeezing through Ospina's legs before he converted past the back-tracking Shkodran Mustafi, who had already slipped over.
The red card was enough to draw a furious reaction from Deigo Simeone, and the volatile former midfielder was also subsequently sent to the stands for the remainder of the game, and may yet be given a ban for next week's second leg at the Wanda Metropolitano.