AC Milan were aided by Sassuolo goalkeeper Andrea Consigli's howler as the Rossoneri moved above city rivals Inter Milan and into third place in Serie A thanks to a 1-0 victory.
Riding an eight-match unbeaten run across all competitions ahead of this encounter at San Siro, Milan had been further boosted the previous day by their neighbours' surprise 2-1 defeat at Cagliari.
And Gennaro Gattuso's side took full advantage as Sassuolo contributed to their 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 visitors' hopes of a comeback as Milan consolidated their position within the UEFA Champions League qualification places thanks to a fourth straight league triumph.
Despite having little to play for, mid-table Sassuolo posed plenty of problems and were the first to threaten, Gianluigi Donnarumma making a fine save to deny Filip Djuricic's glancing touch at the near post.
But it was the hosts 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.
Jeremie Boga curled an effort off the right-hand post as Sassuolo sought a response and the forward did find the net with a fine finish shortly before the break only to see the goal correctly disallowed for offside.
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.
Sassuolo brought on back-up Gianluca Pegolo, who saved smartly from Hakan Calhanoglu soon afterwards and Milan comfortably held on in the closing stages to leapfrog Inter.
What does it mean? Milan boost their top-four hopes
Victory lifts Gattuso's men four points clear of fifth-placed Roma, who face Lazio in the derby later in the day.
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.
Key Opta stats:
- Last time AC Milan were in the top three in the second of the season was back in 2012-2013 on matchday 38.
- Milan have kept clean sheets in each of their opening four home Serie A matches of the year for the first time since 1993.
- Milan have benefited from more own goals (4) than any other team in Serie A this season.
- This was Gennaro Gattuso's 50th Serie A match in charge of Milan (W25 D17 L8). The last Milan boss to have won at least 50 per cent of his first 50 matches was Massimiliano Allegri.
Milan visit Chievo next Saturday evening (Sunday morning AEDT), while Sassuolo play host to second-placed Napoli a day later.