Napoli were given an almighty scare before beating Milan 4-2 at the Stadio San Paolo.
Milan levelled after being 2-0 down at the break, but the visitors ended with nine men as Juraj Kucka and M'Baye Niang were sent off in the closing stages.
The hosts appeared to be coasting to a comfortable Serie A victory when summer signing Arkadiusz Milik scored twice before the interval.
But Niang and Suso produced stunning strikes to suggest that Vincenzo Montella's side can significantly improve on last season's seventh-placed league finish.
Jose Callejon, however, came up with a 74th-minute winner and Milan's frustration was evident when defender Kucka was immediately sent off for picking up a second yellow card.
Niang also saw red three minutes from time and Milan's misery was complete when Alessio Romagnoli put the ball into his own net deep in stoppage time.
Napoli included Kalidou Koulibaly in their starting line-up despite the Senegalese central defender being heavily linked with a move to Chelsea before the transfer window closes on Wednesday.
Koulibaly was quickly tested when Niang crossed from the left and Ignazio Abate should have scored.
But the unmarked Abate somehow contrived to balloon the ball over Jose Reina's cross from six yards out.
Milan's promising start soon petered out as Belgium winger Dries Mertens, the best player on the pitch, began to leave his mark on the contest.
Mertens, who scored twice last week as Napoli recovered from 2-0 down to salvage a point at Pescara, curled a terrific shot on to the post after 18 minutes and Milik was in the right place to slot home the rebound for a first goal since his move from Ajax.
The Poland striker has a heavy scoring burden to carry following the departure of Gonzalo Higuain to Juventus, the Argentina forward having scored 36 league goals for Napoli last season.
But Milik showed his aerial ability after Mertens had forced two wonderful saves from Milan's 17-year-old goalkeeper Gianluigi Donnarumma.
Callejon's corner was directed towards the penalty spot where Milik rose above Kucka to leave Donnarumma stranded with a firm header.
Napoli seemed to be in cruise control, but Niang sent a a low left-foot shot past Reina within six minutes of the restart.
Maurizio Sarri, the Napoli coach, was upset with an incident in the build-up to the goal, but he took his protests with the official too far and was ushered from the dug-out.
Four minutes later Milan were level as Spanish midfielder Suso unleashed a rising 20-yard drive which gave Reina no chance.
Milan had started their campaign last week with an entertaining 3-2 win over Torino and seemed capable of repeating the feat.
But Callejon put Napoli back in front when he prodded home the rebound after Donnarumma had pushed out Mertens' shot.
Milan then lost their discipline with Kucka shoving over Mertens and Niang booked for a second time for stopping Reina throwing the ball out.
And Milan were in defensive disarray again as Romagnoli scored a comedic own goal as he tried to claw the ball off the line with his hand.