The Bianconeri are still looking for their first victory in Serie A and slumped to their second successive defeat since Ronaldo returned to Manchester United, losing 2-1 at the Diego Armando Maradona Stadium.
Kalidou Koulibaly scored the winner in the 85th minute after Moise Kean inexplicably headed the ball towards his own goal following a Napoli corner, setting up a tap-in for the centre-half.
Matteo Politano equalised for the hosts just before the hour mark following Alvaro Morata's opener for Juventus early on.
Massimiliano Allegri's team still have just one point from a 2-2 draw at Udinese on the opening day of the campaign.
Fiorentina made it back-to-back wins with a 2-1 success at Atalanta. All three goals came from the penalty spot, with Dusan Vlahovic scoring from the spot either side of half-time before Duvan Zapata pulled a goal back in the 65th minute.
Venezia, back in Serie A for the first time since 2001-02, gained their first win of the season thanks to goals from Thomas Henry and David Okereke in a 2-1 success at Empoli. Nedim Bajrami's last-minute penalty was mere consolation for the hosts.
Bayern Munich moved up to second in the Bundesliga with an emphatic 4-1 win at last season's runners-up RB Leipzig.
Robert Lewandowski opened the scoring for Bayern from the penalty spot and teenager Jamal Musiala volleyed the second shortly after the break, before teeing up Leroy Sane for the third goal.
Konrad Laimer pulled one back for the hosts but Eric Maxim Choupo-Moting rounded off the scoring for the visitors late on, leaving Leipzig with three defeats in their opening four matches under new coach Jesse Marsch.
Defending champions Bayern are now two points behind leaders Wolfsburg, who maintained their 100 per cent start with a 2-0 victory at Greuther Furth thanks to goals from Lukas Nmecha and Wout Weghorst.
Erling Haaland's double helped Borussia Dortmund come from behind three times to claim a thrilling 4-3 win at Bayer Leverkusen.
Haaland cancelled out Florian Wirtz's opener with a towering header before later netting the winner from the penalty spot.
In a frantic encounter, Patrik Schick and Moussa Diaby scored for Leverkusen but Julian Brandt and Raphael Guerreiro joined Haaland on the scoresheet as Dortmund came out on top.
Goals from Jonathan Burkhardt and Marcus Ingvartsen gave Mainz a 2-0 victory at Hoffenheim, while Rafael Czichos' last-minute own goal rescued a point for Freiburg in a 1-1 draw with Cologne.
The visitors had led through Anthony Modeste's strike before having Florian Kainz sent off after 74 minutes. Union Berlin and Augsburg shared the points in a goalless draw.
Paris St Germain were without Lionel Messi as they maintained their status as the only team in Ligue 1 with a 100 per cent record with a comfortable victory against Clermont.
With Messi watching on in the stands after returning from a controversial international break with Argentina, Ander Herrera's double got the ball rolling in a 4-0 win.
Kylian Mbappe extended the lead early in the second half before Idrissa Gueye completed the scoring after 65 minutes.
Bamba Dieng scored both goals as Marseille maintained their unbeaten start with a 2-0 victory at Monaco.
Quick-fire goals from Dani Vivian and Inaki Williams midway through the second half saw Athletic Bilbao go top of the table in Spain with a 2-0 win against Mallorca.
The other match in LaLiga finished all square as Sergi Guardiola struck late on to rescue Rayo Vallecano a draw at Levante, who had led through Roger's first-half spot-kick.