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.
"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."
Good work by in-form Inter strike duo Martinez and Romelu Lukaku paved the way for the first goal minutes into the game.
But Gosens volleyed in with quarter of an hour to go and Inter goalkeeper Samir Handanovic saved a Luis Muriel penalty two minutes from time to hold onto a point.
Atalanta are fifth, equal on points with Roma, who occupy the final Champions League berth.
In Sardinia, Ibrahimovic was named in the starting 11 for Milan against Cagliari, having come on as a substitute in their goalless draw against Sampdoria last weekend.
The 38-year-old played the entire match scoring after 64 minutes for his first Milan goal since leaving the club for Paris Saint-Germain eight years ago.
"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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Portuguese forward Rafael Leao, 20, had struck the opener just after the break as Milan got their first win since a 3-2 victory over Bologna on December 8.
Since then Stefano Pioli's side have settled for two goalless draws and suffered a humiliating 5-0 defeat by Atalanta.
Milan move up to eighth in the table, 10 points off the Champions League berths.
"I feel good, the coach just wanted to think about my age and see if I needed a rest, but there's no problem," continued Ibrahimovic, who did not want to be substituted.
"My brain is always the same and the body has to follow."
It was his 57th goal for the club and 123rd in Serie A, having also played for Inter Milan and Juventus.
He was denied a second with eight minutes to go by the linesman's flag.
"Ibra is an important point of reference, but it will still take time to perfect," said Pioli.
"He's not yet the real Ibra, which we will see in a few games."
Ibrahimovic has a six-month contract with an option for another season. He helped the club win their 18th and most recent Serie A title in 2011. Cagliari fell to their fourth consecutive defeat, but remain sixth.
- Perfect 10 -
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Lazio extended their winning streak to a club record of ten games as they inflicted a second consecutive defeat on Serie A runners-up Napoli thanks to a blunder from goalkeeper David Ospina with eight minutes to go.
Colombian international Ospina was caught in possession inside his own box allowing Immobile to tap in his 20th goal in 19 games to the delight of the home crowd as the club marked their 120th anniversary.
Lazio are chasing their first Scudetto since 2000.
"What is missing for the Scudetto? To be honest, we're not missing much," said coach Simone Inzaghi.
"We keep conceding crazy goals," said Napoli coach Gennaro Gattuso whose side drop to tenth place.
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