Liverpool began their Champions League title defence with a 2-0 defeat away to Napoli, who scored two late goals to decide a cagey encounter at the Stadio San Paolo on Tuesday.
Dries Mertens scored from the penalty spot with eight minutes remaining after Andy Robertson tripped Jose Callejon inside the box and Napoli substitute Fernando Llorente added a second in stoppage time to put the result beyond doubt.
Juergen Klopp's side became the first competition winners to lose the opening game of their Champions League defence since AC Milan were defeated by Ajax Amsterdam in 1994.
Hirving Lozano headed in from close range early on but the goal was correctly ruled out for offside, before Sadio Mane forced Alex Meret into a save and Roberto Firmino flashed a header wide before the break.
Adrian denied Mertens with a superb reaction stop early in the second half before Meret dived to direct Mohamed Salah's effort wide.
But the late double from Mertens and Llorente secured the three points for Carlo Ancelotti's side, who also beat Liverpool at home at the same stage last season.
In the other Group E game, Salzburg made an impressive start with a 6-2 victory at home to Genk.
Erling Haaland scored a first-half hat-trick for the Austrian champions, becoming the third youngest player to net a Champions League treble at the age of 19 years and 58 days, behind Raul (18y 113d) and Wayne Rooney (18y 340d).