Zlatan Ibrahimovic scored the opening goal and was then later sent-off as AC Milan were beaten 2-1 by city rivals Inter in the quarter-finals of the Coppa Italia, courtesy of a 97th-minute winner from Christian Eriksen.
Veteran striker Ibrahimovic was also involved in an ugly clash with his former Manchester United teammate Romelu Lukaku at the end of the first half.
Ibrahimovic gave his side a 31st-minute lead, drilling in off the post, before tempers frayed in the San Siro.
The Swede and Lukaku locked heads and exchanged words at the end of the first half, before the pair had to be restrained as the players left the pitch for the interval.
The referee cautioned both before giving Ibrahimovic a second yellow card and a red in the second half for a foul on Aleksandar Kolarov.
It was the first time the 39-year-old had been dismissed since 2015, when he was shown a straight red playing for Paris Saint-Germain at Chelsea in the last-16 of the Champions League.
"He [Ibrahimovic] apologised like the great champion he is," Milan manager Stefano Pioli said. "He got a bit caught up in the desire to help the team.
"It [the confrontation] certainly influenced the match, the first booking caused the sending off. They’re big enough and ugly enough, I don’t know what they said to each other, it can happen."
Pioli's Inter counterpart, Antonio Conte said: “I haven’t spoken with Romelu yet, but I was a player so I know that during a match things can get heated. Situations can flare up but it’s only right that things then settle.
"I liked seeing Romelu so focused, he is improving from a character point of view. He plays an important role for us and if sometimes he gets angry, then we can only gain from it.”
Inter will play Juventus or SPAL, who play their quarter-final on Wednesday, in the last four.
The two Milan clubs are also fighting it out in the league, with the Rossoneri leading second-place Inter by two points.
Milan broke the deadlock in the 31st minute when Ibrahimovic controlled the ball on the edge of the area before placing an angled drive between the legs of Inter defender Kolarov and in off the base of the far post.
The yellow card following the Lukaku spat was to prove costly for Ibrahimovic as he was shown another after the break for a tackle on Kolarov, leaving his side down to 10 men.
And Inter were level 13 minutes later. Leao brought down Nicolo Barella and, after viewing the incident again on the pitchside monitor, the referee pointed to the spot. Lukaku slammed the penalty into the top left corner.
The match had a lengthy delay as referee Paolo Valeri received treatment for a thigh injury and was eventually replaced by the fourth official.
Milan goalkeeper Ciprian Tatarusanu kept his team in the game with crucial saves but, just as the match appeared to be heading into extra time, Eriksen curled a free-kick into the top left corner in the seventh minute of stoppage time.
Eriksen’s short-lived Inter career was expected to end this month after club chief executive Giuseppe Marotta confirmed the midfielder is among the players the Nerazzurri are willing to sell. But Conte said the 28-year-old Eriksen remains in his plans.
“He is part of the project,” the former Juventus and Chelsea manager said. "I urged him to take the free-kick, he’s a bit shy but I hope this goal helps him. We all want him to do well because he’s a good lad, perhaps too good.
"We’re keeping a close eye on him as he is part of the group. We’ll all keep working hard right until the end.”
