Douglas Costa was sent off for spitting at an opponent as the Juventus winger's red card marred a 2-1 win in which Cristiano Ronaldo scored his first goals for the Italian champions.
Ronaldo opened his account in the easiest possible fashion by tapping into an empty net from less than two metres in the 50th minute.
The Portuguese added another in the 65th minute when he finished off a counter-attack by drilling a low-angled shot beyond Andrea Consigli and into the far corner with his left foot.
After Ronaldo missed two good chances for a hat-trick, Khouma Babacar headed one back in the 90th minute but could not prevent Sassuolo from falling to their first defeat of the season.
However, Costa became involved in a running feud with Sassuolo midfielder Federico Di Francesco.
The Brazilian escaped with a booking for elbowing and then head-butting his rival as they tangled before the Sassuolo goal, but was then sent off after video cameras caught him spitting at the same player in another altercation.
Juventus, chasing an eighth successive Serie A title, have 12 points, three ahead of Napoli. Earlier, AS Roma squandered a two-goal lead when they were held 2-2 at home by lowly Chievo
Additional reporting by Reuters.
