Walter Mazzarri's side laid out their title credentials in style with a 3-0 success at the San Siro.
A red card for Joel Obi and a dubious penalty decision in the 43rd minute helped Napoli on their way - although Marek Hamsik missed the penalty, Hugo Campagnaro converted the rebound.
Further goals from Christian Maggio and Hamsik in the second half earned Napoli all three points to take them two points clear at the top and leave Inter just one point above the bottom three.
In Saturday's other game, Roma won their second game in a row, beating Atalanta 3-1 at the Stadio Olimpico to end the Bergamo side's unbeaten start to the campaign.
Bojan Krkic opened his account for the Giallorossi in the 20th minute before Pablo Osvaldo added a second after half an hour.
German Denis opened the game up with a goal in the 48th minute, but Fabio Simplicio settled Roma's nerves with the winner nine minutes from the end as Roma moved up to fourth.