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 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.
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."
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.
The 31-year-old Belgian moved back to his former club in Sardinia this season for family reasons.
Inter are the ex-Roma player's favourite victim having scored six career goals against the northern club, four at the San Siro.
Romelu Lukaku missed a late chance for the hosts, sending an effort wide, as both sides pushed desperately to end their winless streak.
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.
"We had dominance in the game and against Inter it's not easy," said Cagliari coach Rolando Maran.
"We take away a well-deserved result, in the way we wanted."