
Napoli improved their mid-table position and did a big favour to second-placed Roma, who Saturday crushed Parma 4-0 to move within six points of the 61-point title-holders with 12 games left.
The injury-plagued Inter fell behind on three minutes at the brimming San Paolo and ran several risks in the remainder of the game as they failed to organise any telling reaction.
Napoli's winner was a lob curled from Uruguay's Marcelo Zalayeta after goalkeeper Julio Cesar only managed a clumsy clearance to block the onrushing Ezequiel Lavezzi.
The hosts did not relent after taking the lead, as the 62,000- strong crowd cheered them on and produced two wide shots with Lavezzi and one with defender Matteo Contini.
Inter were dangerous on their first attack on 19 minutes as David Suazo found goalkeeper Matteo Gianello ready to save his shot from inside the area.
Three minutes later, the hosts' defender Paolo Cannavaro managed to deflect a shot from the young Mario Balotelli, who played up front next to Suazo for the injured Zlatan Ibrahimovic and Julio Cruz.
Napoli had more mouthwatering chances before the break through a low drive from Marek Hamsik and a nasty header from Cannavaro grabbed by the diving Julio Cesar.
The Brazilian was crucial for Inter on 44 minutes diving prodigiously to slap away a botched clearance from Marco Materazzi.
After the break, coach Roberto Mancini brought on Argentina's veteran Hernan Crespo for a three-pronged attack, but Julio Cesar saved Inter again on 53 minutes deflecting a smashing shot from Hamsik.
The custodian's great form was confirmed on 75 minutes as he flew on his left to deflect a spot kick from Zalayeta after causing the penalty with a foul on Uruguayan Walter Gargano.
"I think we are not going through a great phase," Mancini said about a record of two defeats, including a 2-0 at Liverpool in the Champions League, and two draws in the last four games.
"This is due also to the absence of several players. It can happen. We have a good lead (in the Serie A), which now is smaller. We should not lose heart as we will get back some players and do well again."
By Alberto Cagliano, dpa