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 stalemate on Sunday.
Tensions reached boiling point at the San Siro as Inter's goalscorer Lautaro Martinez was sent off in injury time for arguing with the referee.
The Argentinian risks a long suspension after reacting with fury, shouting and repeatedly punching the plexiglass at the side of the pitch, after being led away by his teammates.
Antonio Conte's second-placed Inter are now three points adrift of leaders Juventus, who travel to 13th-placed Napoli later on Sunday.
They can also be caught by Lazio, in third, who are three points behind before their derby clash against fourth-placed Roma.
A win would see Simone Inzaghi's Lazio join Inter in the standings, with a game in hand to be played in February against Hellas Verona.
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Martinez's frustration mirrored that of his team who have now been held to three consecutive 1-1 draws -- at home against Atalanta and Cagliari and at Lecce.
Inter have taken 11 points from the last seven games, not enough to keep pace with eight-time reigning champions Juventus.
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."
- 'Kill games off' -
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"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."