Serie A leaders Juventus suffered a shock 2-1 defeat at Napoli on Sunday, with coach Maurizio Sarri returning to his former club for the first time, though Cristiano Ronaldo still managed to score for an eighth league game in a row for the visitors.
Juve saw their lead cut to three points by Inter Milan, who drew 1-1 at home to Cagliari earlier on Sunday.
Midfielder Piotr Zielinski, who Sarri brought to Napoli during his three-year stint at the club, broke the deadlock in the 63rd minute and captain Lorenzo Insigne hooked in the home side's second goal with four minutes left.
Out-of-sorts Juve took more than an hour to manage a shot on target although Ronaldo pulled one back in the 90th minute as he continued his scoring run.
Sarri, who led Napoli to second place with a club record points tally in his final season in charge in 2017-18, was jeered by the crowd and insulting banners were placed in various parts of the city before the match.