Lazio manager Maurizio Sarri returned to his former playground to snatch a victory courtesy of a 67th minute Matias Vecino strike.
Lazio moved up to second in the Serie A table after inflicting leaders Napoli’s second league defeat of the season.
Matias Vecino’s excllent strike – the first goal Napoli conceded in six games – in the 67th minute from outside the penalty area gave the visitors the priceless triumph.
Serie A top goalscorer Victor Osimhen 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.
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.