Match ends, Sampdoria 3, Napoli 0.
Fabio Quagliarella scored a sensational goal as Sampdoria inflicted a first loss on Carlo Ancelotti as Napoli boss.
Gregoire Defrel scored the first two goals - blasting a fantastic opening effort into the top corner from 20 yards to round off a counter-attack.
He doubled the lead with a deflected effort following a pass from Quagliarella.
Quagliarella sealed the win against his hometown club with a backheel volley from Bartosz Bereszynski's cross.
The hosts brought on England Under-21 midfielder Ronaldo Vieira, a summer signing from Leeds, for his debut with a few minutes remaining.
Last season's runners-up Napoli had won both their opening games under Ancelotti, who replaced new Chelsea manager Maurizio Sarri.
Elsewhere, Sassuolo beat Genoa 5-3 to keep up their unbeaten start and Luis Alberto gave Lazio - who had lost their first two matches - a 1-0 win over Frosinone.
Match ends, Sampdoria 3, Napoli 0.
Second Half ends, Sampdoria 3, Napoli 0.
Arkadiusz Milik (Napoli) wins a free kick in the defensive half.
Foul by Joachim Andersen (Sampdoria).
Foul by Marko Rog (Napoli).
Edgar Barreto (Sampdoria) wins a free kick in the defensive half.
Delay over. They are ready to continue.
Delay in match Allan (Napoli) because of an injury.
Ronaldo Vieira (Sampdoria) is shown the yellow card for a bad foul.
Allan (Napoli) wins a free kick in the defensive half.
Foul by Ronaldo Vieira (Sampdoria).
Corner, Sampdoria. Conceded by Raúl Albiol.
Attempt blocked. Fabio Quagliarella (Sampdoria) right footed shot from outside the box is blocked. Assisted by Karol Linetty.
Attempt missed. Dries Mertens (Napoli) left footed shot from the centre of the box is too high. Assisted by Arkadiusz Milik with a cross.
Substitution, Sampdoria. Ronaldo Vieira replaces Albin Ekdal.
Marko Rog (Napoli) is shown the yellow card for a bad foul.
Foul by Marko Rog (Napoli).
Edgar Barreto (Sampdoria) wins a free kick in the defensive half.
Attempt missed. Karol Linetty (Sampdoria) right footed shot from outside the box misses to the left. Assisted by Gastón Ramírez.
Allan (Napoli) is shown the yellow card.
Foul by Mário Rui (Napoli).
Edgar Barreto (Sampdoria) wins a free kick in the defensive half.
Attempt missed. Arkadiusz Milik (Napoli) right footed shot from outside the box is close, but misses the top right corner. Assisted by Dries Mertens.
Foul by Adam Ounas (Napoli).
Gastón Ramírez (Sampdoria) wins a free kick in the defensive half.
Allan (Napoli) wins a free kick in the defensive half.
Foul by Gastón Ramírez (Sampdoria).
Attempt missed. Jacopo Sala (Sampdoria) right footed shot from outside the box misses to the left following a set piece situation.
Mário Rui (Napoli) is shown the yellow card for a bad foul.
Foul by Mário Rui (Napoli).
Gregoire Defrel (Sampdoria) wins a free kick on the right wing.
Goal! Sampdoria 3, Napoli 0. Fabio Quagliarella (Sampdoria) right footed shot from the centre of the box to the bottom left corner. Assisted by Bartosz Bereszynski with a cross following a set piece situation.
Foul by Kalidou Koulibaly (Napoli).
Gastón Ramírez (Sampdoria) wins a free kick in the attacking half.
Substitution, Napoli. Marko Rog replaces Amadou Diawara.
Amadou Diawara (Napoli) is shown the yellow card for a bad foul.
Foul by Amadou Diawara (Napoli).
Gastón Ramírez (Sampdoria) wins a free kick in the attacking half.
Adam Ounas (Napoli) wins a free kick in the defensive half.
Foul by Jacopo Sala (Sampdoria).