Juventus fell to their third Serie A defeat of the season as Maxime Lopez’s injury-time strike gave Sassuolo a memorable win.
The mid-table visitors led when Davide Frattesi ran onto’s Gregoire Defrel through ball to finish.
Weston McKennie looked to have rescued a point for Juventus with a header from Paulo Dybala’s deep free-kick.
But Lopez ran onto Domenico Berardi’s brilliant crossfield ball and chipped Mattia Perin with almost the last kick.
It was Sassuolo’s first Serie A win at Juventus at their ninth attempt.
Juventus boss Massimiliano Allegri – in his second spell in charge – became only the third man to manage the club in 200 Serie A games, after Giovanni Trapattoni and Marcello Lippi.
But his side drop to eighth, behind Lazio on goal difference, in the table and 13 points behind leaders AC Milan.
This is only the second time since 1996 they have lost three of their opening 10 games. They lost four in 2015-16 – but still went on to win the title.