Sulley Muntari, Urby Emanuelson and Robinho scored in a half-hour spell for the champions and leaders before Daniel Pudil reduced the arrears for the Sea Horses.
Lazio had defender Andre Dias and coach Edoardo Reja sent off as they crashed 5-1 at Palermo, where five players shared the goals for the hosts, and lost third place to Udinese who drew 0-0 at home to Cagliari.
Fabio Borini gave AS Roma a 1-0 win over Parma in another Sunday afternoon match while Chievo beat Genoa by the same score. Novara and Atalanta played out a goalless stalemate.
Milan have 50 points from 24 games, one ahead of unbeaten Juventus who briefly held the lead after their 3-1 win over Catania on Saturday. Juve have a game in hand.
Roma's win took them up to fifth with 38 points, above Napoli and Claudio Ranieri's Inter Milan who were thumped 3-0 at home by Bologna on Friday.
Udinese, who ended a two-match losing streak, are third with 42 points, ahead of Lazio on goal difference.
Novara's draw gave them 17 points and took them off the bottom of the table where they were replaced by Cesena.
Muntari, who joined Milan from arch-rivals Inter in January, opened his account in the 29th minute, the Ghanaian from close range after Thiago Silva's free-kick rebounded into his path.
Emanuelson added the second with a long-range effort as Milan made light of an injury crisis which ruled out Kevin-Prince Boateng, Alessandro Nesta and Alexandre Pato, and the absence of suspended Zlatan Ibrahimovic.
Robinho, partnering Maxi Lopez in attack, broke clear to add the third 10 minutes after the restart before Pudil pulled one back with a half-volley.
Lazio took an makeshift injury-hit team to Sicily and fell behind after 10 minutes when Paraguay midfielder Edgar Barreto scored from the edge of the area.
Massimo Donati also scored from the edge of the area 10 minutes later and Argentine defender Matias Silverstre headed the third just before the break.
Igor Budan and Fabrizio Miccoli added two more within six minutes of the restart and Lazio were in further disarray when Dias was dismissed for a second bookable offence and coach Reja was ordered from the touch-line.
Palermo appeared to ease off and Libor Kozak pulled one back for Lazio with five minutes left.
Udinese faced unadventurous opponents and their best chance came when Gelson Fernandes headed wide with the goal at his mercy just before half-time.
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