Napoli boosted their hopes of finishing second and gaining automatic entry into the Champions League thanks to a bad error by Inter Milan defender Yuto Nagatomo.
The team that finishes third has to go through a playoff for Europe's premier club competition.
Roma, who lost to Lazio earlier in the day, have a more difficult run-in than Napoli. They face AC Milan next and Juventus before ending their season against Chievo Verona and Genoa. Napoli play Cagliari, Torino, Fiorentina and Sampdoria.
Inter have picked up just two points from their past six matches and slipped three points behind city rivals AC Milan, who occupy the last qualifying spot for the Europa League
Napoli, who had earlier hit the post through Dries Mertens, took a deserved lead two minutes from half-time.
Nagatomo looked set to comfortably clear a cross but instead he hooked it over his head, leaving Jose Callejon to slot the ball between the near post and Inter goalkeeper Samir Handanovic.