Sandro Tonali's stoppage time winner against Lazio sent AC Milan back to the Serie A summit, while Napoli's title hopes appear over after their implosion in Empoli.
Ciro Immobile gave Lazio an early lead at the Stadio Olimpico in Rome, but Olivier Giroud levelled shortly after half time before Tonali's late heroics saw Milan leapfrog city rivals Inter, who beat Roma 3-1 on Saturday, into top spot. Inter trail by two points and can go back above Milan if they win their game in hand Wednesday away to Bologna.
Immobile became the leading active scorer in Italy's top flight when he turned in a cross from Sergej Milinkovic-Savic inside four minutes.
The Italy international's 181st Serie A goal and 26th league goal of another prolific campaign took him past veteran Sampdoria striker Fabio Quagliarella.
Milan were level five minutes after the break when French striker Giroud poked in from after excellent work from Rafael Leao, and Milan won it at the death as Zlatan Ibrahimovic nodded across for Tonali to force home following a poor defensive header by Francesco Acerbi.
"My players are lions," said Milan coach Stefano Pioli told DAZN. "I am so proud of my players and if I was in their shoes, I'd be angry, as people don't give them enough credit."
Napoli's Scudetto prospects are hanging by a thread after they blew a two-goal lead late on and crashed to a shock 3-2 loss at Empoli.
With four matches to play this season, Napoli are seven points behind Milan and five adrift of second-placed Inter.
Luciano Spalletti's side were 2-0 ahead with 10 minutes to go before Scottish midfielder Liam Henderson sparked a revival for the hosts who scored three goals in an eight-minute spell.
"Imagining such an end to the match is difficult," said Spalletti, who side have now gone three matches without a win.
"But these are games where the opponent continues to fight with the right attitude, when we probably lost a bit of our focus and made mistakes that we shouldn't make.
"I'm responsible for this team, for its technical and mental attitude, I have to take the consequences for what happens on the pitch."
Champions League qualification does not seem threatened as Napoli still have a nine-point cushion over Roma in fifth.
