Champions Juventus lost for the third time in six Serie A matches this season after goals from Lorenzo Insigne and Gonzalo Higuain secured a 2-1 home win for Napoli on Saturday.
Elsewhere, AS Roma demonstrated their title credentials by trouncing new boys Carpi 5-1 at the Stadio Olimpico to climb up to fourth place on 11 points.
Despite some encouraging moments during the opening 45 minutes, Juve lacked cohesion and looked no closer to ending their malaise as they slipped to 13th with five points.
The Turin giants fell behind after 26 minutes when Insigne scored his third league goal of the campaign in style, playing a one-two with the impressive Higuain before firing low past keeper Gianluigi Buffon from long range.
Insigne was replaced by Dries Mertens 13 minutes later due to injury but the change cost Napoli little of their attacking menace.
They doubled their lead in the 62nd minute when Higuain, quick to intercept a sloppy pass out of defence from Anderson Hernanes, tore down the left before beating a despairing Buffon.
Juve responded immediately as Mario Lemina finished from close range following a cross from Paulo Dybala but Massimiliano Allegri's side were unable to find an equaliser despite the introduction of Juan Cuadrado and Alvaro Morata.
The win pushed mid-table Napoli on to nine points.
Going into Saturday's match on the back of defeat at Sampdoria, Roma started strongly and were 3-0 up after 31 minutes thanks to Konstantinos Manolas, Miralem Pjanic and Gervinho.
Former Roma forward Marco Borriello responded for second from bottom Carpi in the 34th minute but second-half goals from Mohamed Salah and Lucas Digne confirmed a thumping victory for Rudi Garcia's team.