Giacomo Raspadori made Napoli provisional leaders of Serie A with the late winner in Saturday's 1-0 home win over Spezia which put them ahead on goal difference from 10-man AC Milan, 2-1 winners at Sampdoria.
Italy forward Raspadori struck the decisive goal in the final moments of a stodgy encounter at the Stadio Maradona, sweeping home Hirving Lozano's cross to net his first goal for Napoli on his second start after arriving on loan from Sassuolo last month.
"It's a moment that will stay in my head and heart," Raspadori told DAZN.
"I missed two chances which I should have scored but thankfully the team put me in the position to score and I kept my head."
Earlier, Junior Messias gave Milan the lead at Samp after just six minutes when he fired in first time at the end of a neat passing move started by Rafael Leao.
Ten minutes later though, Leao collected a yellow card for raising an elbow and he was dismissed two minutes into the second half when he attempted an overhead kick and caught Sampdoria centre back Alex Ferrari in the face.
Against 10 men, the initiative briefly swung to Sampdoria and Filip Djuricic levelled in the 57th minute when he flicked a near-post header past Milan goalkeeper Mike Maignan.
Milan netted the winner 10 minutes later after Gonzalo Villar handled Olivier Giroud's header from a corner and the Frenchman buried the penalty kick given after a review by referee Michael Fabbri.
Elsewhere, another late strike from Marcel Brozovic gave Inter Milan a hard-fought 1-0 win over Torino to offer some relief from recent blues.