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.
"We were passive for the whole game," said Sarri. "It was a bland performance, we were caught between pressing and waiting. They only had to do the bare minimum to beat us."
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.
Napoli, who are 10th on 27 points and having one of their worst seasons for years, had lost their first three home league games since Gennaro Gattuso replaced Carlo Ancelotti as coach in January.
Sarri fielded an adventurous lineup with Gonzalo Higuain and Paulo Dybala alongside Ronaldo but they were stifled by Napoli's tireless running.
Gattuso, who only a week ago slammed an "embarrassing" home display in a 2-0 defeat by Fiorentina, was much happier than his rival.
"We have to keep going like this now, not let up and focus on Napoli 24 hours a day," he said. "I haven't been here long, but this team have hurt themselves with certain attitudes and behaviour.
"We have been jeered and insulted, often quite rightly, so it's up to us now to turn things around."
Ronaldo put the ball in the net from a rebound in the 53rd minute but it was disallowed for offside against Higuain in the build-up. Juve finally managed a shot on goal in the 62nd minute but Higuain's effort was comfortably saved by Alex Meret.
A minute later, Napoli went ahead. Juve keeper Wojciech Szczesny could only parry a fierce, dipping shot from Insigne and the ball fell to Zielinski who rifled it into the top corner.
Insigne volleyed the second from Jose Callejon's cross but Ronaldo then scored a consolation, getting between two defenders to turn Rodrigo Bentancur's long ball into the net.
Juve nearly levelled in a tense finale but Higuain's overhead kick was saved by Meret.