Inter Milan's perfect start to the Serie A season ended as Sassuolo mounted a second-half comeback to secure a shock 2-1 victory on Wednesday.
Inter still top the standings on 15 points but are level with AC Milan after six matches after Stefano Pioli's side won 3-1 at Cagliari. Sassuolo are eight with nine points.
Denzel Dumfries broke the deadlock for Inter one minute into first-half stoppage time when he cut into the box and unleashed a low shot into the bottom corner of the net.
Sassuolo equalised in the 54th minute when Nedim Bajrami delivered an angled shot from 12 yards that Inter goalkeeper Yann Sommer allowed to trickle over the line.
Domenico Berardi sealed victory for the visitors nine minutes later with a powerful strike from distance into the bottom corner.