Napoli returned to the Serie A summit as Giovanni Simeone's 78th-minute header sealed a 2-1 win over previously unbeaten Milan at San Siro on Sunday.
Milan had 11 shots to Napoli's three in a dominant first half, but the visitors went ahead shortly after the break when Matteo Politano stroked home from 12 yards following Sergino Dest's clumsy foul on Khvicha Kvaratskhelia.
Olivier Giroud pulled Milan level in the 69th minute, but substitute Simeone popped with the winner to seal a huge victory for Luciano Spalletti's side, who remain unbeaten in the league this season.
The result means Napoli leapfrogged Udinese and Atalanta at the top of the table, while Milan dropped down to fifth after a first Serie A defeat in 23 games.
Milan started strongly and were denied a 13th-minute lead when Giroud's strike from 12 yards was superbly tipped onto the crossbar by Alex Meret.
The Napoli goalkeeper was called into action again shortly before the half-hour mark when he pushed over Rade Krunic's header from a corner.
Napoli offered almost no attacking threat in the first half, yet they went ahead 10 minutes after the interval when Politano scored from the spot after substitute Dest had fouled Kvaratskhelia – the decision given after referee Maurizio Mariani was told to check the pitch-side monitor by VAR.
Meret denied Junior Messias soon after, before Giroud pulled Milan level with a simple finish from eight yards after fine work down the left by Theo Hernandez.
Their joy was short-lived, however, as Simeone stole into the area to glance home Mario Rui's cross from the left wing.
Milan should have snatched a point with four minutes remaining, yet substitute Pierre Kalulu inexplicably crashed against the crossbar with only Meret to beat from eight yards.
What does it mean? Milan undone again by Spalletti's men
Milan have now lost their past three home league games against Napoli – the first time this has happened since 1979.
Napoli, meanwhile, have now picked up 10 points from their first four Serie A matches this season. They are unbeaten in the first four away games in an Italian top-flight season for the third campaign in a row – something they have never achieved before.
Politano's penalty was hardly sweetly struck but it still had enough on it to beat Mike Maignan. It was the Italy international's fourth goal against Milan – he has not scored more against any other side in Serie A.
Kalulu's costly miss
Kalulu had the chance to salvage a point for Milan in the dying stages, but the 22-year-old defender got well under his close-range effort and could only watch on as it cannoned back off the crossbar.
Key Opta Facts
- Inter, Juventus, Roma and Milan have lost a Serie A match in a single day for the first time since February 13, 1955.
- Giroud has found the net in three successive league appearances for the first time since July 2020, for Chelsea in the Premier League.
- Prior to Politano, the last Napoli player to score a Serie A penalty away from home against Milan was Maurizio Domizzi on January 13, 2008.
- Simeone scored his fourth Serie A goal as a substitute and his first since February 17, 2019, for Fiorentina against SPAL.
Milan are away to Empoli after the international break, while Napoli host Torino.