Nicola Sansone scored a superb first-half free-kick to earn Sassuolo their first victory over Juventus, who had Giorgio Chiellini sent off on another miserable night for the visitors.
A torrid night saw the Serie A champions slip to their fourth defeat of the season and their third away from home, as coach Massimiliano Allegri saw one of his former clubs outplay his stuttering side in the rain at Mapei Stadium.
Allegri’s men paid for their physical approach to the game, as poor tackles led to the decisive free-kick from which Sansone scored, as well as the dismissal of experienced defender Chiellini.
Sassuolo's home form has been at the heart of their excellent start to the season, and Eusebio Di Francesco's side are now unbeaten in eight home league matches – a run that stretches back to April.
Allegri, meanwhile, must address his side's away form; the champions have won only once on their travels in Serie A this season.
Juve started the night unbeaten in their previous five outings, and they enjoyed a spell of early pressure.
Paulo Dybala sent a free-kick bending towards goal that forced Gianluca Pegolo to make an awkward punched clearance, with the damp conditions making life difficult for the Sassuolo goalkeeper who was playing in place of the suspended Andrea Consigli.
The steady rain led to some heavy challenges, and Mario Lemina was booked for a clumsy tackle that gave Sassuolo a free-kick in a dangerous position after 20 minutes. Sansone stepped up and curled in an inch-perfect shot that evaded both the Juve wall and the reach of Gianluigi Buffon.
Juve looked to Paul Pogba for inspiration and the Frenchman responded, creating space for himself to launch a dipping shot at goal that drew a save from Pegolo at full stretch.
Just as the visitors seemed to be finding their feet, Chiellini clattered into Domenico Berardi and, having already booked him earlier in the half, referee Andrea Gervasoni had no hesitation in dismissing the veteran defender.
Despite their man disadvantage, the champions came out with intent after the break, and Dybala crossed for Mario Mandzukic, whose effort was blocked but he laid the ball off for Pogba to slam a shot narrowly wide.
Allegri sent on Alvaro Morata for the misfiring Mandzukic as Juve continued to press for an equaliser, and his introduction seemed to energise Dybala, who sent a fizzing shot just wide of Pegolo's goal.
Berardi had the ball in the net in the closing stages, but the linesman's flag denied him a goal and Sassuolo a more emphatic first win over the Bianconeri, whose title defence suffered a further blow.
