The first half of the Milan derby was not all sunshine and roses for the visitors, who lost 2-1 on aggregate.
In a game that should have been a draw, they were outplayed for most of the first 45 minutes, despite scoring twice from two men, but were unable to take advantage.
“We tried to play from behind and we didn’t have enough quality, but we played good football and created chances,” said Milan striker Dries Mertens.
“The first half was just a bit of fun, it was a good start to the season.”
The visitors had an early opportunity when Milan’s Dani Alves had an effort tipped on to the bar by substitute Davide Santon.
However, the ball was saved by goalkeeper Marco Callejon.
Merten could not find the net in the second half as Milan failed to convert their first chance of the evening, when Diego Godin headed in a free kick.
In the third period, the visitors had chances to win the game, with Alessandro Florenzi finding space in the middle of the park before shooting wide of goal.
Mereu, however, kept up the pressure.
He was denied by Callejón in the 68th minute, but the referee’s decision meant that Milan were awarded a penalty after Calleja’s initial touch.
In the 89th minute the hosts were awarded their second penalty, when Di Marzo was adjudged to have fouled Florenze in the box.
Florenza made a good save to deny Di Marzone, but Callejo did not see it coming.
The third goal came after a minute of stoppage time.
Milan’s Diego Calleoni was denied an early free kick by goalkeeper Gianluigi Donnarumma, who saw his shot from the edge of the box go wide of the far post.
After a goalmouth scramble in the final third, Mertesini took a touch and curled a shot from close range into the top corner.
Milan had to be content with two points from their opening four games of the season.
“I don’t think we played well in the last game,” Mertsini said.
“It was a difficult game, but now we have to start to focus and find a rhythm.
It’s good to get the points, but it is not over yet.”