Napoli were beaten 1-0 by Parma on their own turf on Saturday in their second consecutive loss after falling to Juventus two weeks ago.
Former Italy forward Antonio Cassano scored the winner for Parma in the 81st minute with an impressive solo run.
Dries Mertens then threatened for Napoli in the 89th minute with a long-range free kick that skimmed the top of the crossbar.
Third-place Napoli remained four points behind leaders Roma, who host Cagliari on Monday, and three points behind Juventus, who visit Livorno on Sunday.
Inter Milan lost ground on the leaders as they were held 1-1 at struggling Bologna.
The visitors fell behind to Panagiotis Kone's 12th-minute goal, with Jonathan levelling in the 51st minute. The point left Inter in fourth place, two points behind Napoli and eight behind Juventus.
Mario Balotelli failed to convert a penalty as AC Milan were held to a 1-1 draw by 10-man Genoa at the San Siro to extend the Rossoneri's winless streak in Serie A to five matches.
Kaka put the hosts ahead four minutes in with an expert control and shot but former Milan forward Alberto Gilardino equalised four minutes later with a penalty.
In the 36th minute, Thomas Manfredini was sent off for a foul on Balotelli, but Balotelli's ensuing spot kick was too central and was saved by Genoa goalkeeper Mattia Perin.Earlier, bottom club Chievo Verona beat Hellas Verona 1-0 with a last-minute goal in a Serie A derby that hadn't been played in 11 years.
Left unmarked inside the area, Slovenian midfielder Dejan Lazarevic scored with a powerful shot in the second minute of added time for his first Serie A goal.
It was Chievo's first match since Eugenio Corini was rehired as coach. The victory moved Chievo provisionally level with Sampdoria and Catania while promoted Hellas remained sixth.
Information from The Associated Press was used in this report.