Veteran Swedish star Zlatan Ibrahimovic scored on his first AC Milan start since his return to the club in a 2-0 win over Cagliari on Saturday but city rivals Inter's position on top of Serie A is at risk after a 1-1 draw against Atalanta.
Lautaro Martinez put Inter ahead after four minutes in the San Siro only for Robin Gosens to pull Atalanta level on 76 minutes.
Inter are just one-point clear of champions Juventus who play Roma in the Stadio Olimpico on Sunday.
Lazio consolidated third position, four points behind Inter, with the league's top scorer Ciro Immobile grabbing the only goal against Napoli to stretch his side's winning streak to ten games.
Atalanta striker Duvan Zapata got his first start after a long injury layoff but Inter were struggling without suspended Nicolo Barella and injured duo Matias Vecino and Alexis Sanchez.
"We gave everything in a difficult situation," said Inter coach Antonio Conte.
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"We were down to the bare bones, without four big players.
"Looking at our position in the table, seeing us up there, we might turn a blind eye to some issues.
"But I hope we don't have any more missing players or the situation becomes problematic."
"I'll celebrate like a God at San Siro, not here," said Ibrahimovic, who last scored in the Milan jersey in May 2012. "I celebrate God every time I score, that way I feel alive!"
- 'The real Ibra' -
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"My brain is always the same and the body has to follow."
"He's not yet the real Ibra, which we will see in a few games."
- Perfect 10 -
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