Stefano Pioli made five changes from the Rossoneri side that drew with Juventus and the Grifone swiftly took advantage in the opening five minutes.
Fikayo Tomori conceded the penalty that allowed Genoa to find the back of the net for the first time in four league head-to-heads via Retegui, who confidently sent his spot-kick beyond Marco Sportiello.
Christian Pulisic then threatened to restore parity, impressively curling an effort that kissed the right post on its way out.
Aiming for a second successive clean sheet and a first at San Siro since 2017, the visitors did well to keep Milan’s forwards at bay for much of the first period, although goalkeeper Josep Martínez could do nothing to stop the hosts from levelling on the verge of HT.
Connecting perfectly with Samuel Chukwueze’s cross, Alessandro Florenzi headed home the equaliser from point-blank range to net his first goal of the season and draw the scores level.
Just as they did in the first half, Genoa almost instantaneously reclaimed the lead after the restart, with Caleb Ekuban earning plaudits for a calm header inside the right post, giving Sportiello no chance of saving the effort after receiving the ball from Alessandro Vogliacco.
With 20 minutes left, the Rossoneri turned the game on its head, scoring twice in quick succession to take the lead for the first time.
Matteo Gabbia first headed an unstoppable shot from inside the six-yard box before Olivier Giroud smashed home a fantastic volley following a pass from the right wing to potentially win at home for the first time in three games.
However, against the odds, the Rossoblu kicked it up a notch in the final few minutes, ultimately stealing a draw thanks to a defensive mishap from Milan, with Retegui in the right place to tap home an equaliser from close range.
Former Rossoneri striker Alberto Gilardino will now prepare his side to take on US Sassuolo, AS Roma, and Bologna in their last three games as they push for a top-half finish.
Milan, on the other hand, have little to play for, although they are yet to confirm their runners-up spot after a sixth successive winless outing across all competitions.
Flashscore Man of the Match: Mateo Retegui (Genoa)
