Juventus' dreadful start to their Serie A title defence continued as Gonzalo Higuain inspired Napoli to a 2-1 victory at the Stadio San Paolo on Saturday.
After providing the assist for Lorenzo Insigne's first-half opener, Higuain – who was left out of Gerardo Martino's latest Argentina squad – scored the second for Maurizio Sarri's men shortly after the hour mark.
Insigne fired Napoli ahead with his third Serie A goal of the season 26 minutes in, following a frantic start to proceedings that saw Pepe Reina make an excellent save to deny Simone Zaza.
Giorgio Chiellini should have headed Juve level just after the restart and the Serie A champions were made to pay for sloppy play from Hernanes when Higuain stole in to net his fifth league goal of 2015-16.
Mario Lemina opened his Juve account, but Massimiliano Allegri's side were unable to find an equaliser, with substitute Alvaro Morata going closest.
The defeat is Juve's third in their opening six games and they could be 13 points adrift of league leaders Inter should Roberto Mancini's side beat Fiorentina on Sunday.
Allegri demanded a response from Juve following their 1-1 draw with Serie A debutants Frosinone in midweek, but it was Napoli who started the stronger – with Higuain blazing over before Marek Hamsik forced Gianluigi Buffon into action.
Napoli, who failed to score in a league game for the first time in 14 Serie A fixtures when they drew 0-0 with Carpi last time out, continued to press, but it was Juve who went close next, Reina doing well to get down low to his right and parry away Zaza's strike.
But the hosts maintained their control, and took a deserved lead when Insigne linked up with Higuain before drilling a low finish beyond Buffon from the edge of the area.
However, the diminutive winger's night was cut short prior to the interval, an apparent knee injury forcing Sarri to introduce Dries Mertens in his place.
And the Belgium winger almost had an immediate impact when he burst in behind Juve's defence, but he elected to shoot rather than play in the unmarked Jose Callejon, and his effort sailed wide.
Reina would have been helpless to prevent Chiellini equalising after the interval had the Italy defender managed to direct his header on target from Paulo Dybala's free-kick.
After seeing an effort well saved by Buffon, Higuain had the goal his performance deserved in the 62nd minute, firing home from an acute angle after dispossessing Hernanes.
But Juve responded immediately, Lemina, on loan from Marseille, tucking home after latching onto Dybala's cross at the back post.
Morata replaced Dybala with 20 minutes remaining and the Spain striker was just inches away from restoring parity with a venomous effort from 18-yards out, but Juve slipped to another crushing setback.
