Liverpool's Champions League campaign got off to a nightmare start in hot and humid Naples on Wednesday as sensational Napoli ran out comprehensive 4-1 winners.
Victor Osimhen set the precedent early on, hitting the post inside the opening 60 seconds before James Milner clumsily conceded a fifth-minute penalty, which Piotr Zielinski kept his cool to convert.
Liverpool, stunned into submission, then presented Osimhen with his own chance to test Alisson from 12 yards after an ill-timed challenge from Virgil van Dijk was penalised by VAR review.
Alisson denied the Napoli striker's spot-kick attempt, but Liverpool's reprieve only lasted 13 minutes. Khvicha Kvaratskhelia outmuscled Joe Gomez, who endured a torrid 45 minutes before being hauled off, to present Andre-Frank Zambo Anguissa with an opportunity to slot the ball low past Liverpool's helpless 'keeper.
Things went from bad to worse shortly before the interval as substitute Giovanni Simeone - on in place of the injured Osimhen - was found unmarked in the six yard box to turn home Kvaratskhelia's pinpoint cross.
Team news - Salah, Diaz start; Osimhen passed fit
Liverpool made three changes as Jurgen Klopp's men kicked off their Champions League Group A campaign at Napoli.
Andy Robertson, James Milner and Roberto Firmino came in following Saturday's derby draw with Everton as Kostas Tsimikas, Fabio Carvalho and Darwin Nunez dropped out.
Thiago Alcantara was named on the bench after a month out, while Victor Osimhen was deemed fit to start for Napoli.
Zielinski rounded off the scoring for Luciano Spalletti's new-look side two minutes into the second period to compound Liverpool's misery, before Luis Diaz temporarily quietened the deafening home crowd when cutting inside to bend a low right-footed shot beyond Alex Meret.
Diaz, Liverpool's only saviour on an otherwise sultry evening, also saw a thumping header saved while concerns in midfield personnel were slightly assuaged by the reemergence of Thiago and debut of Juventus loanee Arthur - both second-half substitutes.
Liverpool overwhelmed in hostile Naples
Liverpool have started their Premier League campaign with a stuttering two wins from six games and carried that languid form into their Group A opener against Serie A's second-placed side.
In contrast, Napoli are unbeaten in five domestic matches and are scoring goals for fun, so it was no surprise when Liverpool's leaky defence opened the door to both Osimhen and Zielinski inside the first quarter of a particularly one-sided game.
Osimhen's surge to beat Liverpool's offside trap and dribble around Alisson before striking a shot off the outside of the post set the tone as the visitors were humiliated 102 days on from losing the European final to Real Madrid in France.
The celebrations are guaranteed to continue long into the night in southern Italy, while Liverpool will hope their weak start to this season has bottomed out following an embarrassing defeat.
Liverpool have lost all three of the games they have played at Stadio Diego Armando Maradona under Klopp - this being their joint-heaviest defeat in Champions League competition.
Klopp: Goals handed on a plate
Liverpool's Jurgen Klopp speaking to BT Sport:
"It was really tough to take but I would say not that difficult to explain.
"Firstly, Napoli played really well and we didn't. That is the first explanation for the defeat. The two penalties we concede, they were both a little unlucky.
"The next two goals we concede are handed to them on a plate. That is not cool and we should have defended better in the first place and, of course, in the last moment as well.
Insipid Reds caught cold
Liverpool suffered the heaviest defeat by an English Premier League side in their opening game of a Champions League campaign since Arsenal lost 3-0 to Internazionale in 2003-04. It was the Reds’ joint-heaviest defeat overall in the competition.
"We were not compact defensively or offensively. Until Thiago entered the pitch I cannot remember one counter-pressing situation. The reason for that is that we are just too wide.
"You could see it on the pitch. We were not working as a team. That is why we lose games. We play in the strongest league in the world and have a tough Champions League group too.
"We have to play in three days against Wolves. If Wolves saw that game tonight they would not stop laughing and would say it is the perfect moment."
Robertson: 'Open' Liverpool deserved to lose
Liverpool's Andy Robertson speaking to BT Sport:
"We were miles too open. You cannot come to a place like this and not be compact.
"They were the miles better team. When you are a team that presses and someone misses a challenge, you need someone to follow it up.
"Too many times, they got turned and had open space to run at, causing our centre-backs problems, whether it was two versus two or three versus two.
"In a Champions League game, you cannot be wide open like that, you have to be compact. We deserved this result. It felt like they had an extra man."
September 13: Ajax (H) - kick-off 8pm
October 4: Rangers (H) - kick-off 8pm
October 12: Rangers (A) - kick-off 8pm
October 26: Ajax (A) - kick-off 8pm
November 1: Napoli (H) - kick-off 8pm
Liverpool return to Premier League action on Saturday as they host Wolves at 3pm, while Napoli will look to maintain their unbeaten start in Serie A as they host Spezia on Saturday in a 2pm kick-off.