Leonardo Bonucci opened the scoring after 20 minutes with a rare long-range effort, but Jeremie Boga tied things up just two minutes later. Francesco Caputo gave the visitors the lead early in the second half before Ronaldo kept his team's unbeaten run going with a penalty goal.
The Bianconeri had won their last six in all competitions.
Juventus manager Maurizio Sarri made a few changes to his side, starting Emre Can, Gianluigi Buffon and Gonzalo Higuain over Blaise Matuidi, Wojciech Szczesny and Paulo Dybala.
Sassuolo made the brighter start, however, pushing forward with numbers and taking control of the ball. It didn't lead to any immediate chances, although Buffon had to be alert to keep out a free-kick from Giorgos Kyriakopoulos.
Boga fired a shot over the bar after 18 minutes, and almost immediately after, the Bianconeri took the lead on the other end of the pitch. It was centre-back Bonucci, of all people, who found himself closer to goal than usual and decided to let fly from outside the box:
Their advantage didn't last long, however. Just two minutes after Bonucci opened the scoring, he failed to track his man, and without any help defence from Can, Boga was wide-open to dink a finish over Buffon.
ESPN's Janusz Michallik was loving the action:
Juventus increased the pressure after the equaliser, but Sassuolo's defence was barely troubled. Higuain fired a shot over the bar, and Rodrigo Bentancur had an effort blocked. Ronaldo was virtually invisible for the bulk of the half.
Improvements were needed, but things only got worse early in the second half. Juan Cuadrado created trouble for Matthijs de Ligt, and his clearance went straight at Caputo, who beat Buffon far too easily with a rather tame strike.
Per sports writer Karl Matchett, it was an all-round disaster at the back for the defending champions:
Ronaldo finally showed himself shortly after, but his free-kick was saved brilliantly by the 18-year-old Stefano Turati, who was making his Serie A debut. The teenager followed that up with another excellent save, denying Higuain.
Ronaldo finally beat the debutant after 60 minutes, rounding him before netting, but the offside flag went up when he was already celebrating.
The former Real Madrid man did get his goal after 68 minutes, converting a penalty after former Juventus prospect Filippo Romagna took out Dybala inside the box. Marco Messina of Italian Football TV praised La Joya for winning the spot-kick:
The Bianconeri pushed for a winner, but Dybala could not beat Turati from a promising position. Miralem Pjanic also went close, blasting a free-kick narrowly over the bar.
Sarri made a last gamble, introducing Aaron Ramsey in favour of Higuain, and the Welshman nearly beat Turati shortly before the final whistle. The last chance of the match fell to Ronaldo, who fired narrowly wide.
Juventus visit Lazio Roma on Saturday. Sassuolo host Perugia in Wednesday's Coppa Italia action.