Arsenal captain Martin Odegaard's sensational late strike broke stern Napoli resistance as the English champions began their bid for Champions League glory with a hard-fought 1-0 win in Naples on Wednesday.
Mikel Arteta's side, who lost to Paris Saint-Germain on penalties in last season's final, spurned several glorious chances against the well-drilled hosts before the Norwegian lashed a shot off the post and in from the edge of the area with 15 minutes to go.
It continued a perfect start to the campaign for Arsenal, who began their Premier League title defence with three wins in a row before condemning Massimiliano Allegri's team to a third consecutive defeat.
Arsenal's next European test comes at home to Lille on Tuesday, October 13, while Napoli travel to Villarreal on the same day as they look to bounce back.
Injuries to Scott McTominay and Andre-Frank Zambo Anguissa forced Allegri into a midfield reshuffle as Kevin De Bruyne and Billy Gilmour came in, while left-back Mathias Olivera was also selected after the hosts let a two-goal lead slip to lose 3-2 away to Inter Milan on Saturday.
Arteta made four changes from Arsenal's 2-1 win over Chelsea on Sunday as Piero Hincapie, Mikel Merino, Eberechi Eze and Viktor Gyokeres earned their first starts of the season.
The hosts set out to soak up pressure and frustrate last season's runners-up, who struggled to break the low block early on and were almost caught out at the other end when De Bruyne and Matteo Politano both called goalkeeper David Raya into action.
Arsenal carved out a big chance midway through the first half when Merino's header from eight yards out was pushed over the bar by Alex Meret.
The Gunners, who won all eight of their league phase games to kickstart their run to the final last season, continued to probe as Ben White produced a tame finish from a promising position before Meret scooped a Hincapie cross-shot off his line.
Rasmus Hojlund's hold-up play was proving a useful weapon for Napoli on the counter, as Politano forced another save from Raya.
But it was Arsenal who ended the first half feeling they should have been in front after Saka was played through on goal and had only the keeper to beat, only to lift a poor finish over the bar.
NAPOLI RESISTANCE BROKEN
Napoli were forced into a goalkeeper change during the interval as Vanja Milinkovic-Savic replaced the injured Meret, but Arsenal picked up where they left off as Hincapie wasted another good chance by scooping his shot over from five yards.
Arsenal continued to knock at the door to no avail until the 75th minute, when in-form Odegaard smashed an unstoppable shot in off the post from the edge of the area, just three days after scoring a winning strike against Chelsea.
The visitors had a huge chance to settle it with five minutes to go when Noni Madueke had a shot charged down by Milinkovic-Savic before fellow substitute Christos Tzolis's follow-up effort was blocked on the line.
They were almost made to pay when a neat passing move from Napoli left De Bruyne with space in the box, but the Belgian opted to play a ball across an empty six-yard box instead of shooting.
Kai Havertz thought he had doubled the Gunners' lead in stoppage-time with a tap-in from Milinkovic-Savic's parry, but the offside flag was up.