Holders Liverpool began their defence of the Champions League trophy on a false note after going down 2-0 to Napoli on Tuesday.
Liverpool survived a 1-0 defeat in Naples in the group stage last season to go on and win their sixth European Cup, and this time a 2-0 reverse at the San Paolo made them the first reigning champions to lose their opening game in the competition since 1994.
Both sides had chances but the Group E clash was edging towards a goalless conclusion until a late penalty award for the hosts, awarded when Jose Callejon went down under an Andy Robertson challenge.
Dries Mertens beat Adrian from 12 yards and substitute Fernando Llorente – who played for Tottenham Hotspur against Liverpool in last season’s final – then took advantage of a rare defensive lapse by Virgil van Dijk to make it 2-0 in stoppage time.
Liverpool must now bounce back in two weeks when they host Austrian champions Salzburg, who made a stunning return to the group stage as teenage forward Erling Braut Haaland netted a hat-trick in a 6-2 demolition of Genk.
Last season’s Europa League winners Chelsea ensured it was two defeats out of two on the night for English sides as they went down 1-0 at home to Valencia.
The Spaniards scored in the 74th minute when a Dani Parejo free-kick was expertly converted by Rodrigo Moreno.
Frank Lampard’s men were then given a chance to rescue a draw when Daniel Wass was penalised for handball in the box, but Ross Barkley’s 87th-minute penalty hit the bar on its way over.
Also in Group H, last season’s semifinalists Ajax claimed an impressive 3-0 home win over Lille with two of their newest signings – Quincu Promes and Edson Alvarez – among the goals.
After a patchy start to the season, Barcelona will be relieved to escape from their trip to Borussia Dortmund with a 0-0 draw as Lionel Messi returned from injury as a second-half substitute.
The home side will feel they should have won, but Marco Reus had a second-half penalty saved before Julian Brandt hit the bar late on.
Those two appear well placed already to progress from Group F after Inter Milan needed a stoppage-time Nicolo Barella goal to rescue a 1-1 draw against rank outsiders Slavia Prague earlier.
Nigerian forward Peter Olayinka put Slavia ahead in the 63rd minute, only for Barella to level in the second minute of stoppage time after a Stefano Sensi free-kick hit the bar.
In Group G, RB Leipzig won 2-1 at Benfica with Timo Werner bagging a brace before Haris Seferovic pulled one back for the hosts.
Earlier, Memphis Depay’s penalty gave Lyon a 1-1 draw at home to Zenit Saint-Petersburg after Iranian striker Sardar Azmoun had opened the scoring.