Juventus remain without a victory this season after they lost to Napoli in the side's first game since Cristiano Ronaldo left for Manchester United.
Ronaldo, who helped Juventus win Serie A twice in three seasons, rejoined United in a £12.8m deal on transfer deadline day and scored twice for them in Saturday's 4-1 win over Newcastle.
In Ronaldo's absence, ex-Chelsea striker Alvaro Morata gave Juve an early lead, but goals from Matteo Politano and Kalidou Koulibaly helped Napoli come from behind to win.
Koulibaly's winner came with five minutes to go after an error from Moise Kean, playing for Juventus for the first time since completing a two-year loan move from Everton.
Kean had only been on the pitch for three minutes and nearly scored an own goal with a misplaced header from a corner before Koulibaly converted the rebound, after goalkeeper Wojciech Szczesny could only parry Kean's header.
Juventus, who came fourth in 2020-21, have only picked up one point from their opening three Serie A matches.
Napoli are top of the league with nine points from three games, although they will lose top spot if Lazio, Roma, Inter Milan or AC Milan win on Sunday.
"It is a good start. I have to thank Spalletti because he has put together a team and given them a very interesting style of play," said Napoli owner Aurelio De Laurentiis. "We were always on the attack, controlling the game from start to finish."
Napoli play away at Leicester City in their Europa League opener on Thursday but may be without Italy striker Lorenzo Insigne after he injured himself while taking a corner and had to be replaced.