Daichi Kamada handed Napoli the first defeat of their Serie A 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 debutant Matteo Guendouzi, who was excellent as a second half substitute, ruled out for offside.
"Everyone put themselves on the line... tonight we showed what we could do against the best team in Serie A," said Luis Alberto to DAZN.
"It wasn't an easy week for us or for me and this brilliant display helped us to shake off a few things."
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 bottom three.
Inter Milan can draw level with their local rivals with a home win over Fiorentina on Sunday evening.
- Napoli flop -
Napoli were on top in the early stages but it was Lazio who opened the scoring on the half hour mark thanks to a bit of magic from Luis Alberto.
The Spanish midfielder stunned the home fans with a cheeky backheeled finish from close range after great work down the right form Felipe Anderson.
Lazio didn't stay ahead for very long as two minutes later Zielinski's speculative effort was deflected past Ivan Provedel.
But instead of a second half onslaught from Napoli it was Lazio who came out the brighter team and Kamada soon had his first Serie A goal.
Anderson was again involved, bursting down the right before laying across the penalty area a pass which Luis Alberto dummied for Kamada, who took two touches and drilled across Provedel into the bottom corner.
Five minutes later Zaccagni thought he had extended his team's lead but failed to hold his run before charging onto France international Guendouzi's pass and rolling past Alex Meret.
And it was Zaccagni again in the 73rd minute who strayed offside for Luis Alberto's pass just before Guendouzi lashed in what he thought would be his first Lazio goal after moving to the Italian capital on loan from Marseille on Thursday.
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 a 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.