I Bianconeri were unbeaten in 22 league games and cruising heading into the game at the Stadio San Paolo but goals from Jose Callejon and Dries Mertens gave third-placed Napoli the 2-0 victory. Rafael Benitez's men may be a distant 17 points off the championship pace, yet Roma have a game in hand and could cut Juve's lead to only eight points by beating Parma on Wednesday.
Roma trimmed the gap to the defending champions back to 11 points on Sunday afternoon with a 2-0 victory over relegation-threatened Sassuolo. I Giallorossi took a half-time lead through Mattia Destro before Michel Bastos added a second deep into second-half stoppage time to ensure a fourth straight win for Rudi Garcia's men.
In the race for the UEFA Europa League spots, fourth-placed Fiorentina drew 0-0 at Sampdoria while sixth-placed Parma went down to a last-gasp goal at Lazio, who moved to within two points of their opponents with the 3-2 victory.
Italy winger Antonio Candreva decided the outcome with a stoppage-time strike. Earlier, Parma forward Jonathan Biabiany had cancelled out Senad Lulic's opener before Michael Ciani's own-goal brought the visitors level once again following Miroslav Klose's second-half effort.
Luca Toni scored twice as ten-man Hellas Verona snapped a run of four straight Serie A defeats with a comfortable 3-0 victory over Genoa - despite playing half the match with a man less. Verona had not won - or scored - since beating Livorno on February 23 to drop down to tenth, but they ended that run thanks to Marco Donadel's first-half goal and Toni's late brace. Toni's goals came despite the fact Verona had been playing with ten men since Michaelangelo Albertazzi's dismissal on the stroke of half-time.
Also, tenth-placed Torino took full advantage of Cagliari's woeful away record to claim a 2-1 triumph. Goals in either half from Omar El Kaddouri and Alessio Cerci set up Torino's win, before Nene grabbed a rare away goal for Cagliari.
The Sardinians have not won away in Serie A since February last year - a run of 21 games - and their arrival was welcomed by a Torino side that had seen their European ambitions falter after five defeats in their past six games.