AC Milan were handed a stark reminder of how much work they still have to do to return to the upper echelons in Serie A after Sassuolo claimed a famous 2-1 victory at San Siro.
Filippo Inzaghi's men harboured aspirations of climbing into the European places on Tuesday and Andrea Poli put them on the right track in the early exchanges with his first goal of the season.
Ambitious visitors Sassuolo were in no mood to roll over, however, and two assists from Domenico Berardi helped Gianluca Sansone and then, after the break, Simone Zaza to turn the game around.
The surprise result leaves seventh-placed Milan two points shy of the last Europa League berth while Sassuolo - in only their second top-flight campaign - climb into the top half of the table.
In noticeably high spirits following their festive friendly win over European champions Real Madrid in Dubai, the Rossoneri started brightly and took the lead after only nine minutes.
Giacomo Bonaventura did well to make up ground on the right flank before crossing the ball to Stephan El Shaarawy. The young Italy striker was tackled but Poli tucked home the rebound.
Milan failed to capitalise on their advantage, though, allowing Sassuolo to steadily turn the screw as the game neared the half-hour mark.
Alex did well to take the ball off Sansone's feet in the 25th minute but three minutes later he would manage to notch.
The Germany-born forward controlled Berardi's sumptuous aerial pass before beating Diego Lopez with a smooth half-volley.
Sassuolo should have taken the lead before the break but Zaza could not reach Simone Missiroli's pinpoint cross and Sansone saw his low effort on Berardi's assist bobble past the far post.
The second half failed to spark until the introduction of Alessio Cerci -- recently signed on loan from Atletico Madrid -- drew cheers from the San Siro crowd.
He was to be upstaged in some style, though, as Zaza fired Sassuolo into the lead with aplomb.
After a swift counter-attack earned the visitors a corner, the shaven-headed striker met Berardi's inswinging ball with his left boot and volleyed it between Lopez and the near post.
Giampaolo Pazzini entered the fray as Milan searched for a late equaliser and he nearly grabbed it in the 75th minute - only the quick reflexes of Andrea Consigli kept the former Inter Milan man at bay.
ends