Liverpool began the defence of their Champions League crown on a losing note as Dries Mertens' penalty and Fernando Llorente's injury-time strike handed Napoli a 2-0 victory.
On a balmy night at the intimidating Stadio San Paolo, the Reds fell behind in the 82nd minute when Mertens (our anytime scorer tip at 2/1) converted from the spot after Andy Robertson had upended Jose Callejon in the area.
Substitute Llorente - part of the Tottenham side beaten by the Reds in last season's Champions League final - then took advantage of a mistake from Virgil Van Dijk to bag his first goal for Napoli after joining them earlier this month.
Liverpool have started their Premier League campaign with five successive victories but largely struggled for composure in their Group E opener, while they lacked a clinical edge on a night where they were barracked throughout by a boisterous home support.
They were indebted to a couple of crucial saves from goalkeeper Adrian for keeping the scores level and, though Roberto Firmino and Mohamed Salah went close for the visitors, their errors in the later stages proved costly.
This was their second defeat in Naples inside a year, having gone all the way in Europe's elite club competition despite losing all three matches away from Anfield in the group stage last season.
Liverpool boss Jurgen Klopp said on BT Sport: "This was a much better game than we played last time when we lost 1-0 in the last minute (to Napoli last season). It was an open game, in moments we controlled the game, but we had a lot of counter-attacks and didn't finish them off.
"That's a problem. Then in the second half we had this wild game, we were running, they were running. They've killed everybody on the pitch, started controlling again and then we've conceded a penalty. Of course that doesn't help and I've seen it now, I don't think that's a penalty.
"For me it's clear and obvious, no penalty, because Callejon jumps before he has any contact. But we cannot change that so we have to be critical with ourselves. We did well in a lot of moments, we played a lot of good football but we didn't finish off.
"We controlled it in moments, but we had not enough chances in the end. What you saw was a lot of respect from both sides, we both defended well and each ball we lost, or they lost, was immediately a threat.
"The last ball is the most difficult ball anyway, and if you are not in the right circumstances it's even more difficult. And so there were moments when we should have done better, for sure."