Luis Suarez's last-gasp penalty gave Atletico Madrid a 2-1 comeback win on Tuesday at AC Milan, who failed to hold on to their early lead after playing for an hour with 10 men.
Seven-time European champions Milan thought they were on for their first Champions League win since 2013 when Rafael Leao opened the scoring with 19 minutes gone in the Group B clash at the San Siro, but Antoine Griezmann struck with a superb volley with six minutes left to level the scores before Suarez's stoppage-time spot-kick broke Milan hearts.
Diego Simeone's side sit second in the group on four points, two behind Liverpool, after a win which looked unlikely until Franck Kessie was sent off in the 30th minute for a second bookable offence.
"Eleven against eleven, Milan were better than us, they played with more rhythm and were more dynamic. They pressed better than us and forced us to not play the way we wanted to," Simeone told Sky Sport Italia.
"When the red card came they fell back and that allowed us to get back into a match in which, up until that point, we weren't in the game at all."
Milan were on top until then, snappy in the tackle and on top of Spanish champions every time the away side had the ball, but sit bottom with zero points following their second successive defeat.
"We're disappointed because we were close to a really important result, a positive result against a really good team," said Milan coach Stefano Pioli to Sky.
"We played much better than them when it was 11 against 11 and that should make us more convinced of our qualities and our way of playing.
"It's a shame because the boys put in a really great effort and coming out of this game beaten makes things more complicated for us."
- Suarez downs Milan -
There was a feverish atmosphere at the San Siro, which was hosting Milan's first Champions League home match since the 2013 group stage.
Brahim Diaz was at the heart of Milan's attacking play and should have had a fine assist to his name in the 19th minute when he sent Ante Rebic through on goal, only for the Croatia forward to put his finish too close to the onrushing Jan Oblak.
Diaz was involved again when the hosts took the lead a minute later through Leao. The Portuguese forward followed neat footwork in the box from his Spanish teammate by arrowing a low shot into the opposite corner of Oblak's goal.
Milan looked comfortable against an underwhelming Atleti but Kessie handed the visitors a lifeline when having already been booked, arrived late into a challenge on Koke and was sent off.
Despite the disadvantage Milan held their own and Leao came close to doubling their lead in the most spectacular fashion, crashing a 38th-minute bicycle kick off the bar from the edge of the area before being flagged for the tightest of offsides.
However Atleti slowly began to force Milan back into their own area. Luis Suarez had two great chances to level either side of the break, first volleying wide on the stroke of half-time and then heading Joao Felix's pinpoint cross the wrong side off the upright in the 54th minute.
Milan held on resolutely and looked to be on their way to victory until Griezmann met Renan Lodi's knock back with a beautifully-timed volley which gave Mike Maignan no chance.
The home fans filled the San Siro air with whistles, but with the clock ticking down Pierre Kalulu was judged to have handled in the area after a VAR check. Suarez did the rest to earn Atletico a precious win with his first Champions League away goal since 2015.