Inter got the three points thanks to a Romelu Lukaku penalty, but Napoli largely dominated the game even when down to 10 men after Lorenzo Insigne saw red for dissent.
There was only one point separating these sides, and that was the point docked from Napoli for the cancelled trip to Juventus, a matter that is still subject to appeal.
Arturo Vidal, Alexis Sanchez, Radja Nainggolan and Matias Vecino missed out for the hosts, while Victor Osimhen was still suffering from a dislocated shoulder.
Napoli had started the brighter of the two teams, but when trying to keep the ball in play for a cross, Dries Mertens landed awkwardly and seemed to sprain his ankle as he went down. The Belgian was in tears getting treatment and the ankle was visibly swollen.
Moments later, Lautaro Martinez fired inches wide after Nicolò Barella intercepted a poor Kalidou Koulibaly pass.
Andrea Petagna replaced Mertens and tried to set up Piotr Zielinski, but the Poland international drilled off target.
Romelu Lukaku nodded wide from a couple of yards, but it wouldn’t have counted anyway for an earlier Lautaro Martinez push.
The second half tempo was also still sluggish and when Hirving Lozano did get a great chance to shin it over, he was probably offside anyway.
Napoli thought they had finally taken the lead with a sensational Insigne back-heel flick, but Samir Handanovic somehow managed the reaction save from point-blank range.
Moments later, the game turned on its head. David Ospina stuck up a hand and leg to catch Matteo Darmian for a clear penalty. Insigne was infuriated and sent off for insulting the referee, so Napoli were down to 10 men and Romelu Lukaku converted the penalty to put Inter in front.
Napoli weren’t to be deterred and Lozano pulled it back for Matteo Politano, but Handanovic made an extraordinary flying save on his former teammate under the bar.
It was impossible to tell Napoli were down to 10, as they continued to get the best chances, Politano seeing another attempt deflected onto the side-netting and Handanovic forced to use his foot to deny Giovanni Di Lorenzo from point-blank range.
Petagna went even closer, turning his marker to fire against the base of the upright in stoppages.
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Sent off: L Insigne 72 (N)