Serie A leaders Napoli suffered only their second league defeat of the season as they lost at home to Lazio, who moved up to second in the table.
Matias Vecino scored the only goal in the 67th minute with an excellent strike from outside the penalty area.
Serie A top goalscorer Victor Osimhen, with 19 goals in the league, came closest to an equaliser but saw his header bounce off the crossbar.
Lazio’s Sergej Milinkovic-Savic also hit the woodwork with a free-kick.
Vecino’s goal was the first goal Napoli had conceded in six games.
Napoli are aiming to become Italian champions for the third time with their last success coming in 1990.
Despite this loss, they are still in a dominant position as they are 17 points clear of Lazio, who move one point above Inter and AC Milan with the Milan clubs both having a game in hand.
AC Milan play at Fiorentina on Saturday before Inter entertain Lecce on Sunday.