The Reds – whose only previous defeat in all competitions this season came against Napoli in September – wanted a victory to secure top spot in Group E with a match to spare.

Yet it was Napoli which took the lead thanks to Dries Mertens' composed 21st-minute finish.

Roberto Firmino missed two big chances to equalise, though it was Dejan Lovren who proved the unlikely hero for the Premier League leader with a towering header as the points were shared in Klopp's 100th European match as a coach.

Liverpool was dealt a blow in the 19th minute, Fabinho unable to continue after sustaining an apparent ankle injury in a collision with Hirving Lozano.

Georginio Wijnaldum replaced his stricken team-mate, but Napoli took full advantage of some disorganised Liverpool defending – Mertens keeping his cool when one-on-one with Alisson after latching onto Giovanni Di Lorenzo's long ball.

VAR checked the legality of Napoli's opener on two counts, first for a potential foul from Mertens on Virgil van Dijk and then for offside, but the goal was allowed to stand.

James Milner was denied by Alex Meret before half-time, though Kalidou Koulibaly had to spare his goalkeeper's blushes when he hooked Firmino's effort off the line.

Though Firmino squandered another fine chance just after the hour, Lovren made no mistake two minutes later as he headed in from James Milner's corner – Mertens' appeals for a foul falling on deaf ears.

And though there was no late winner for Liverpool on this occasion, Napoli failed to muster another opening at the other end as their winless run under Carlo Ancelotti was stretched to seven games in all competitions.