AC Milan suffered a major blow to their Serie A title hopes as Matteo Politano's second-half strike earned Napoli a 1-0 win on Sunday to leave the hosts nine points behind leaders Inter Milan in second place.
The Italian winger's scuffed shot into the bottom corner after 49 minutes sealed a precious victory for the visitors' Champions League qualification hopes, as they moved above AS Roma into fifth place on goal difference and two points behind Atalanta in fourth.
Milan struggled to break down a resolute Napoli defence and their frustration showed when substitute Ante Rebic was sent off for dissent in stoppage time.
Stefano Pioli's side remain second on 56 points, but are now at risk of being dragged into the battle for a top four finish as Juventus are one point behind them in third and Napoli in fifth have 50 points and a game in hand.
"We missed a chance as it could have been an exceptional week for us," Pioli told Sky Italia.
"Inter have done very well since they were knocked out of Europe, but we're just thinking about earning as many points as possible."
Napoli coach Gennaro Gattuso travelled to San Siro as an opposing manager for the first time after a hugely successful spell as a Milan player and two years as coach there between 2017 and 2019.
Milan looked to have turned around their flagging form last week with two impressive away results, beating Hellas Verona 2-0 before earning a 1-1 Europa League last-16 draw against Manchester United.
Piotr Zielinski twice threatened for the visitors in the first half before Politano latched onto a pass from the Polish midfielder to fire in the decisive goal after the break.
Fabian Ruiz's vicious half volley required a strong parry from Gianluigi Donnarumma and Milan's appeals for a penalty in the dying minutes were waved away after referee Fabrizio Pasqua consulted the pitchside VAR monitor and judged that Tiemoue Bakayoko had not fouled Theo Hernandez.
Tensions boiled over when Pasqua heard something he did not like from Rebic and flashed a red card at the Croatian.