Milan crashed to a halt and Genoa emerged from crisis, as Blerim Dzemaili scored and Alessio Romagnoli saw red.
The Rossoneri were fresh from two heart-stopping 3-2 victories over Palermo and Udinese, while the Grifone had lost four of their last five games. Andrea Bertolacci returned from injury to face his former club, moving Jack Bonaventura into the trequartista role, while Carlos Bacca was rested. Goran Pandev and Issa Cissokho sat out bans with Mattia Perin, Alassane Tambe, Ezequiel Munoz, Tino Costa and Cristian Ansaldi sidelined.
Genoa beat Milan home and away last season, interrupting an otherwise disastrous record against the Diavolo of two points from eight meetings.
After five minutes a free kick was worked for Diego Capel to drill through the wall, taking a deflection on the way wide of the post.
It was a similar situation that saw Genoa take the lead, as this time Blerim Dzemaili’s scorching free kick took a deflection off Bonaventura to wrong-foot Diego Lopez.
Bonaventura blasted over the bar and Mario Balotelli’s glancing header from a corner whistled wide, while Capel’s angled drive was straight at Diego Lopez.
Another Dzemaili free kick hit the wall, but the umpteenth Cristian Zapata misplaced pass sent Capel running and Romagnoli clattered into him for a second bookable offence. It left Milan down to 10 men just before half-time and a goal down at Marassi, not to mention a painful return here for ex-Sampdoria defender Romagnoli. Nigel de Jong was sacrificed to introduce centre-back Rodrigo Ely.
Curiously, this was the second time that the Rossoneri had a man sent off before half-time, as Ely saw red against Fiorentina in the opening round.
After the restart, Balotelli’s disguised pass sent Luiz Adriano through for an angled drive beaten away low by Eugenio Lamanna.
There were timid penalty appeals when Bonaventura’s effort struck Giovanni Marchese on the arm, while Balotelli curled a free kick off target and turned another into the wall.
Leonardo Pavoletti threatened to double Genoa’s lead, firing over under pressure from Zapata on a Diego Laxalt assist.
It was difficult to tell Milan were down to 10 men, but at the same time Genoa suffered little at the back in an even second half.
Juraj Kucka came off the bench and had the chance to be the hero against his old club, but incredibly ballooned over the bar from 12 yards with just four minutes to go.
With the last kick of the game, Ely nodded a set play wide of the target.
Sent off: Romagnoli 43 (M)