Milan - Radja Nainggolan grabbed the equaliser in a 1-1 draw for Cagliari to dent the title ambitions of his parent club Inter Milan, who struggled to a third consecutive draw on Sunday.
But Lazio failed to take advantage of another slip up from Antonio Conte's side as their club record 11-match winning run ended with a 1-1 draw against city rivals Roma at the Stadio Olimpico, leaving them third, two points behind second-placed Inter.
Inter are three points adrift of leaders Juventus, who travel to struggling Napoli later on Sunday.
At the San Siro, tensions reached boiling point as Inter's goalscorer Lautaro Martinez was sent off in injury time for arguing with the referee.
The Argentinian reacted with fury, shouting and repeatedly punching the plexiglass at the side of the pitch, as he was led away by his teammates.
Martinez's frustration mirrored that of his team, who have now been held to three consecutive 1-1 draws and taken 11 points from the last seven games.
Inter made a promising start with Ashley Young having an immediate impact on his debut after arriving from Manchester United, providing the cross which allowed Martinez to head in after half an hour.
But the hosts failed to build on the momentum and the Sardinians stayed in the match until Nainggolan's deflected long-range equaliser on 78 minutes.
"On the one hand I'm sorry to have scored," said Nainggolan.
"I have a lot of respect for Inter, the fans and for my teammates, but on the other no, I'm happy. They treated me a little badly, as if I was a mediocre player."
Nainggolan scored the decisive goal for Inter against Empoli in the final game last season to snatch the last Champions League berth at the expense of city rivals AC Milan.
Romelu Lukaku missed a late chance for the hosts, sending an effort wide, as both sides pushed desperately to end their winless streak.
'Kill games off'
Tensions were already high when Martinez went down in a challenge and earned a yellow card for a comment to the referee, promptly earning a straight red for then shouting at the official.
The game ended in chaos with Conte arguing with match officials, before storming off.
"It's definitely not the fault of the referee for the draw, we only complained because no foul was called for our strikers," said captain Samir Handanovic.
"The annoyance always arises when you don't win, we always want to win.
"Something is definitely missing from the team because we keep conceding second half goals, and don't kill games off when we have the chance."
Martinez, who has scored 11 league goal this season, now risks missing the Milan derby on February 9.
Cagliari, in sixth, occupy the final Europa League berth, despite taking just three points from their last seven games.
Lazio failed to take second place from Inter after Roma ended their winning streak, with both goals the result of goalkeeping errors.
Roma's early dominance paid off after 26 minutes when Lazio goalkeeper Thomas Straskosha misjudged Bryan Cristante's long ball for Dzeko, with the Bosnia anticipating the stoppers' exit, nodding the ball in with the back on his head.
But Lazio pulled level eight minutes later after a Pau Lopez shocker, with the Roma goalkeeper unable to clear an aerial shot and a mix-up with Chris Smalling allowing Francesco Acerbi to tap in.
Paulo Fonseca's side remain fourth, one point ahead of Atalanta, who thrashed Torino 7-0 on Saturday.
Lazio have a game in hand to be played in February against Hellas Verona.