Sassuolo midfielder Vasilije Adzic scored a last-gasp winner against his parent club to consign Juventus to their first defeat of the season after a five-goal thriller on Sunday, while Napoli broke a three-match losing streak to beat Bologna.
Edon Zhegrova twice drew Juve level at the Mapei Stadium, including scoring within two minutes of Kieron Bowie's 89th-minute strike, but the Turin club were caught on the counter as Adzic steered home the winner with the last kick of the game.
The 20-year-old Montenegrin, who joined Sassuolo on loan from Juventus in the summer, sealed a victory that sent his side level with the Bianconeri on seven points.
Visiting coach Luciano Spalletti faced an injury crisis, with nine first-team players out, and the starting side he picked made history as the first Juventus line-up in Serie A history not to feature a single Italian outfield player.
Nico Gonzalez produced the best chance of the first half inside two minutes when his shot clipped the top of the bar, before Juventus goalkeeper Guglielmo Vicario had to be alert to charge down Domenico Berardi's finish.
But the game burst into life after the break and Sassuolo took the lead in the 65th minute when Nemanja Matic slipped in Sebastiano Esposito, who kept his composure to find the bottom corner on his first start for his new club.
Zhegrova profited from a goalkeeping error to level with eight minutes to go, but Scotland international Bowie looked to have won it for the hosts after coolly finishing off a counterattack in the 89th minute.
The action was far from done, however, as Zhegrova levelled again with a powerful finish before Juventus were caught out at the death when pushing forward in search of a winner as Adzic struck against his parent club.
NAPOLI END LOSING RUN
Napoli's losing run came to an end as Stanislav Lobotka scored to earn a 1-0 Serie A win over Bologna and ease pressure on coach Massimiliano Allegri.
The Slovakian playmaker's cushioned volley midway through the second half sealed a deserved win for the hosts in Naples.
It was badly needed after Napoli picked up just three points from their first three league games – the club's worst start in 11 years – before a midweek Champions League defeat to Arsenal.
Napoli moved up to ninth place on six points, while Bologna's wait for a first win of the season goes on as they remain in 16th with one point from four games.
"The results didn't come because we lacked a bit of anger," Allegri said.
"Today too, we conceded some crosses late in the first half where we could have paid for it. We need to improve on this aspect. With the same anger and desire, I don't know how the game would have ended against Inter and Como (3-2 and 2-1 losses)."
The Stadio Diego Armando Maradona was forced into stunned silence inside seven minutes when Arthur Theate looked to have headed the visitors in front, but the strike was ruled out for offside.
Kevin De Bruyne could only fire over from a promising position and Rasmus Hojlund came close with a glancing header as the hosts searched for an opening in response.
The breakthrough came in the 66th minute when halftime replacement Costantino Favasuli whipped a low cross into the box that was expertly guided home by Lobotka.
Hojlund was then denied by 18-year-old goalkeeper Massimo Pessina as Napoli pushed for a second.
Bologna coach Domenico Tedesco could only watch on from the stands due to suspension as his side threatened an equaliser late on through Samuel Mbangula, but they failed to score for the third time this season.
Earlier, Lecce beat winless Monza 3-2 for their second victory of the season to move into 11th place.
The round concludes when champions Inter Milan host Udinese on Monday, after Roma travel to Torino and Como face Parma.