Napoli striker Marcelo Zalayeta scored the only goal of the game to end Internazionale's unbeaten record in Serie A on Sunday night.The Uruguayan netted after three minutes and also missed a second-half penalty as Inter finally tasted defeat in their 26th league game of the season.
The setback trimmed the Nerazzurri's lead to six points over second-placed Roma, who thrashed Parma 4-0 on Saturday.
Alberto Aquilani, Francesco Totti and Mirko Vucinic scored for the Giallorossi to add to an own goal from Giulio Falcone.
On Sunday, Pablo Daniel Osvaldo scored a last-minute winner as Fiorentina won 3-2 at Juventus to move to within a point of their third-placed opponents.
With AC Milan held to a 1-1 draw by Lazio on Saturday night, Fiorentina's win moved them four points clear of the Rossoneri in the race to finish in the Champions League qualifying spots.
The top four sides in Serie A will play in Europe's elite club competition next season.
In a pulsating encounter, the Viola went ahead through Massimo Gobbi after 19 minutes only for Mohamed Sissoko to equalise 10 minutes later.
Mauro Camoranesi then put Juve 2-1 up before two Fiorentina substitutes changed the course of the game in a dramatic final 15 minutes.
Ndiaye Papa Waigo equalised within nine minutes of his introduction before Osvaldo, a 62nd-minute replacement for Tomas Ujfalusi, netted in the final minute.
Argentina-born Osvaldo's joy turned sour, however, when he was then sent off after being shown a second yellow card for removing his shirt as part of his celebration.
Udinese strengthened their claims for a UEFA Cup place with a 2-0 win at home to Atalanta - Fabio Quagliarella and Antonio Di Natale grabbing the goals in the first half.
Sampdoria and Torino played out a 2-2 draw in Genoa.
Gianluca Comotto and David Di Michele twice put Torino ahead, the latter goal coming from the penalty spot, with Luigi Sala and Antonio Cassano levelling for the hosts.
Cassano, on loan from Real Madrid, was shown a late red card for swearing at the referee, after which he lost his composure and hurled his shirt at the official.
Livorno and Siena were the big winners in the race to remain in Serie A.
The Leghorn club climbed out of the bottom three thanks to a 1-0 win over Catania, who replace them in the drop zone - Alessandro Diamanti netting the only goal - while Siena's resurgence continued with a 2-0 victory at fellow strugglers Empoli.
Daniele Portanova put the visitors in front and Christian Rigano doubled their advantage in the final minute.
The hosts finished with 10 men after Davide Moro was dismissed late on.
Bottom side Cagliari also won, 2-1 against Genoa, to revive their hopes of staying up.
Alessandro Lucarelli put Genoa 1-0 up after 14 minutes but Robert Acquafresca equalised before an own goal from Genoa goalkeeper Fernando Rubinho won it for the Sardinians, who are now just four points from safety.
Reggina are second bottom following their goalless draw at home to Palermo.
On Saturday, Milan needed a 71st-minute penalty from Massimo Oddo to rescue a point against Lazio, who had gone ahead at the San Siro thanks to Rolando Bianchi.
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