Liverpool opened their Champions League title defence with a 2-0 loss at Napoli on Tuesday, while Barcelona had goalkeeper Marc-Andre ter Stegen to thank for a goalless draw against Borussia Dorrtmund.
Chelsea, however, were not so fortunate as Ross Barkley missed a penalty as the London side went down 1-0 to Valencia at Stamford Bridge.
In Naples, Dries Mertens converted a penalty eight minutes from the end of an entertaining match after a foul by Andy Robertson on Jose Callejon.
Substitute Fernando Llorente then sealed it in stoppage time to gain a measure of revenge after playing on the Tottenham side that lost last season’s final to Liverpool.
“Napoli are a good side and you have to defend as a team which I felt we did,” Reds skipper Jordan Henderson told BT Sport.
“The mistakes for the goals, we need to improve on. Overall we can be better, we take responsibility and try and react the right way.”
Napoli and Salzburg, who routed Genk 6-2, took the early lead in Group E.
Keeper Ter Stegen saved a 57th-minute penalty from Dortmund captain Marco Reus and denied the home side with a string of saves on his return to his native Germany.
Dortmund substitute Julian Brandt struck the crossbar and Paco Alcacer also went close against his former side.
Lionel Messi came on as a substitute for the last 30 minutes for his first appearance of the season but was unable to spark the visitors’ struggling attack, with Dortmund creating the best chances of the game.
“It was a tough game,” Ter Stegen told Sky TV. “The result is OK. I try to play at my best and if I can help the team it is even better.”
In London, Rodrigo gave Valencia a winning start in a 1-0 win over Frank Lampard’s Chelsea.
The Brazilian forward was left unchecked to race onto Daniel Parejo’s free kick and volley into the net in the 74th minute.
Chelsea fans were cheering minutes later when a penalty was awarded after a VAR review determined that Fikayo Tomori’s header hit the hand of Daniel Wass.
But the celebration was short-lived as Barkley’s kick clipped the crossbar and flew over.
In other matches, last season’s semi-finalists Ajax defeated Lille 3-0, Inter Milan secured a stoppage-time 1-1 draw against Slavia Prague, RB Leipzig won 2-1 at Benfica and Olympique Lyon and Zenit St Petersburg drew 1-1.