Napoli suffered their first defeat of the Serie A season as they were beaten 1-0 by Atalanta in Bergamo.
Maurizio Sarri's side were unbeaten in the league going into the match, but now sit four points adrift of leaders Juventus, who won 3-0 at Empoli earlier on Sunday.
The winning goal came in the ninth minute when Andrea Petanga smashed in his third goal of the season from six yards after Alejandro Gomez's cross was poorly dealt with by the away defence.
Atalanta could have doubled their advantage when Boukary Drame smashed a powerful long-range effort against the crossbar before the break, while home goalkeeper Etrit Berisha made saves from Jose Callejon, Faouzi Ghoulam and Arkadiusz Milik at the other end.
Napoli, though, were unable to turn their dominance of possession into goalscoring chances in the second half, with Berisha not called into action as Sarri's men suffered their first defeat in any competition since April.
Following the loss, Napoli have only picked up one victory from their four away league matches in 2016-17.
Atalanta's second straight win moves them on to nine points from seven games.