Domenico Berardi stole the show as his hat-trick gave Sassuolo a deserved 3-2 win over a struggling AC Milan side at the Mapei Stadium.
Milan goalkeeper Diego Lopez was furious at the decision to award Sassuolo's opening goal, which came when he fumbled a 25-yard strike from Berardi.
Lopez recovered from his initial error to claw the ball away from the line, but referee Marco Guida awarded the goal on the advice of his assistant, despite replays appearing to show that the ball did not cross the line.
It was no more than Sassuolo deserved, however, as Eusebio Di Francesco's side dominated the opening period, beating their more illustrious opponents to nearly every ball and attacking with a verve that was absent from much of Milan's play.
Their superiority was rewarded on the half-hour mark when Berardi latched onto Simone Missiroli's long ball forward with a perfectly-timed run that exposed Milan's high line, before drilling the ball beyond Lopez and into the far corner from the edge of the box.
Going two goals behind spurred Milan into life, though, and they pulled a goal back instantly through midfielder Giacomo Bonaventura, who finished his mazy run past two defenders with a smart low finish.
Keisuke Honda came close to restoring parity shortly before the break with a curling strike from the edge of the box that drew a diving save from Andrea Consigli.
Yet it was another goalkeeping error that allowed Milan to draw level just after the restart as Alex easily beat Consigli to a Suso cross and headed into an empty net.
Milan's fightback was dealt a blow when Bonaventura was given his marching orders after collecting a second yellow card for a careless handball, leaving Filippo Inzaghi's side a man down for the last half an hour.
The dismissal turned the tide again as Sassuolo appeared the most likely team to find a winner, with only Lopez atoning for his earlier error with a point-blank save from Berrardi's strike keeping Milan in the tie.
However, Lopez was powerless to stop Berardi completing his hat-trick and winning the game for Sassuolo with a close-range finish from Simone Zaza's flick across the six-yard box.
Milan's misery was compounded when Suso was given a straight red card in added time for a lunging tackle on Francesco Magnanelli.
The result lifts Sassuolo up from 16th to 12th in the Serie A table, just three points behind Milan who sit in 10th place.
