Daichi Kamada handed Napoli the first defeat of their title defence on Saturday with the winning goal in Lazio's eventful 2-1 win in Naples.
Japan midfielder Kamada lashed in his first goal for Lazio since signing from Eintracht Frankfurt this summer in the 52nd minute to give Lazio their first points of the season.
Luis Alberto netted Lazio's opener in a deserved win which lifted Maurizio Sarri's side from the relegation zone and into mid-table.
Lazio were unlucky not to win by more at the Stadio Diego Armando Maradona as they had two further goals from Mattia Zaccagni and Matteo Guendouzi ruled out for offside.
Piotr Zielinski levelled for Napoli almost immediately after Luis Alberto's 30th minute opener but the hosts did very little once Kamada put Lazio in front.
Defeat for Rudi Garcia's team allows AC Milan to stay alone at the top of Serie A thanks to their third straight win, 2-1 at struggling Roma on Friday night.
Doubling satisfying for Lazio is that their impressive win at the home of last season's Scudetto winners also drops Roma into the relegation zone.
Inter Milan can draw level with their local rivals with a home win over Fiorentina on Sunday evening.
Level on six points with Napoli are Atalanta, who saw off Monza 3-0 thanks to Gianluca Scamacca's first goals for his new club.
Scamacca signed for Atalanta from West Ham for reported 25 million euros ($27.4 million) this summer and netted either side of half-time with a towering header and a precision low finish.
The 24-year-old, starting for the first time for Gian Piero Gasperini's side, was not named in Luciano Spalletti's first Italy squad on Friday.
Earlier Bologna came back from a goal down to beat promoted Cagliari 2-1 while Udinese were held to a goalless draw by Frosinone.