Napoli at the double to leave Liverpool stunned
Napoli 2-0 Liverpool
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 Stadio San Paolo, the Reds fell behind in the 82nd minute when Mertens 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.
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 captain Jordan Henderson said: "In the first-half, I felt we played some good stuff, we were just missing that final bit. Napoli are a good side and you have to defend as a team which I felt we did. The mistakes for the goal, we need to improve on.
"Overall we can be better, we take responsibility and try and react the right way."
Jurgen Klopp's side are the first winners of the competition to lose the opening game of the following campaign since AC Milan in 1994. Belgian forward Mertens scored from the penalty spot after Robertson was judged to have fouled Jose Callejon.
Former Tottenham forward Llorente then capitalised on a mistake from van Dijk in stoppage-time to roll the ball past Adrian.
The former West Ham stopper had earlier made two keys saves for Liverpool, who initially struggled to impose themselves in the intimidating atmosphere of the San Paolo Stadium.