AC Milan was aided by Sassuolo goalkeeper Andrea Consigli's howler as the Rossoneri moved above city rival Inter Milan and into third place in Serie A thanks to Saturday's 1-0 victory.
Riding an eight-match unbeaten run across all competitions ahead of this encounter at San Siro, Milan had been further boosted on Friday by its neighbour's surprise 2-1 defeat at Cagliari.
And Gennaro Gattuso's side took full advantage as Sassuolo contributed to its own downfall, Pol Lirola putting through his own net before, midway through the second half, Consigli came out of his area but misjudged the flight of a long ball and hauled down Krzysztof Piatek.
Consigli was shown a straight red card, which effectively ended the visitor's hopes of a comeback as Milan consolidated its position within the Champions League qualification places thanks to a fourth straight league triumph.
It was the host who forged ahead 10 minutes prior to the interval as Suso's right-wing corner took a flick off Manuel Locatelli and, with Mateo Musacchio and Piatek in close attention, deflected in off the unfortunate Lirola.
Consigli's moment of madness arrived in the 64th minute, the keeper seeing Franck Kessie's long pass bounce over his head before he instinctively reached out to grab Piatek's left shoulder.
What does it mean? Milan boosts its top-four hopes
Victory lifts Gattuso's men four points clear of fifth-placed Roma, who face Lazio in the derby later on Saturday.
Poacher Piatek causes panic
He may not have extended his Serie A scoring streak to a fifth game, but Piatek's chasing of Kessie's long pass prompted Consigli to rush hastily from his line and, when he sensed he would not reach the loose ball, the striker ensured his fall would see his opponent punished.
Consigli consigned to an early bath
The visiting keeper could do little about the goal that ultimately decided the game, but his grave error in judgement hamstrung his team-mates' efforts to salvage at least a draw.
Milan visits Chievo next Saturday, while Sassuolo plays host to second-placed Napoli a day later.