Marcelo Zalayeta capitalised on an early blunder by Inter Milan keeper Julio César as Napoli ended the leaders's unbeaten record in Serie A on Sunday with a 1-0 home win.
Zalayeta chipped into an empty net in the third minute after Cesar, who saved a penalty from the Uruguayan striker in the second half, tapped an intercepted long pass to him instead of hoofing away.
Inter still have a six-point lead over AS Roma, who are on 55 points from 26 games after thrashing Parma 4-0 on Saturday.
Earlier on Sunday Fiorentina beat third-placed Juventus 3-2 to move within a point of the Turin side in the standings.
Fifth-placed AC Milan are four points adrift of the Florence club and the Champions League qualification zone after being held 1-1 by Lazio at the San Siro on Saturday.
Napoli continued to dominate after going ahead and César repelled efforts by Marek Hamsik and Paolo Cannavaro before saving Marco Materazzi's blushes before halftime when he palmed the defender's mishit attempted clearance off the line.
The Brazilian keeper was busy again in the second half, pushing away a well-struck Hamsik shot and saving the 75th minute spot-kick, conceded for a foul he had committed on Walter Gargano.
"We aren't going through an exceptional period of form, in part because we have lost lots of players (through injury) at the same time," Inter coach Roberto Mancini told Sky television.
"I'm still optimistic. It can happen that you lose a match or you don't play well. But six points are six points. We'll see what happens next week."
In Turin, Fiorentina, without their injured top-scorer Adrian Mutu, took a deserved lead in the 19th minute when Massimo Gobbi slipped the ball home after a one-two with Martin Jorgensen.
Momo Sissoko levelled in the 29th with an overhead kick after Fiorentina keeper Sebastien Frey made a mess of dealing with a free kick. It was the Mali midfielder's first goal since joining Juve from Liverpool in January.
Italy international Mauro Camoranesi floated unmarked behind the Fiorentina defence to volley home a Cristian Molinaro cross 12 minutes into the second half.
Senegalese striker Papa Waigo N'Diaye came off the bench to restore parity with a diagonal strike and provided the cross for the winner by Pablo Osvaldo, who was sent off for a second yellow card after waving his shirt in celebration.
Antonio Cassano issued a public apology for protesting wildly and shouting abuse at the referee from the sidelines after being sent off for dissent in the closing stages of Sampdoria's 2-2 draw with Torino.
The temperamental striker had scored the goal that earned his side a draw.