Giacomo Raspadori made Napoli provisional leaders of Serie A with the late winner in Saturday's 1-0 home win over Spezia.
AC Milan moved level on points with the leaders, after winning 2-1 at Sampdoria later in the evening despite playing almost the whole second half with 10 men.
In Naples, Italy forward Raspadori struck the decisive goal in the final minute of a stodgy encounter at the Stadio Maradona, putting Luciano Spalletti's side one point ahead of Atalanta.
The 22-year-old swept 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."
Raspadori was one of three signings made in quick succession by Napoli just after the season started alongside Giovanni Simeone and Tanguy Ndombele.
He lead their attack in the absence of starting striker Victor Osimhen after the Nigeria forward was ruled out until at least after the international break. Napoli, who will lose their league lead should Atalanta beat promoted Cremonese on Sunday afternoon, will make his signing permanent next summer assuming he meets unannounced sporting objectives.
They are on a high after a strong start to the domestic season and thrashing Liverpool in the Champions League opener midweek.
They travel to Rangers on Tuesday before their first true title test against AC Milan at the San Siro next weekend. Spezia sit 14th on five points after their third defeat from the first six games of the campaign.