Liverpool's first game in the Champions League since 2014 ended in a draw with Sevilla at Anfield.
In a repeat of the 2016 Europa League final, which the Spaniards won in Basel, Argentine midfielder Joaquin Correa cut through the Liverpool defence and curled home a cool finish to clinch a point.
It had taken Sevilla just five minutes to take the lead as Wissam Ben Yedder tapped in from close range after home defender Dejan Lovren fell over while trying to clear the ball.
Sadio Mane tested visiting goalkeeper Sergio Rico with a low drive as Liverpool looked to get back into the game, and they managed to do so when Roberto Firmino finished off a fine team move from close range.
The five-time winners continued to push forward and were rewarded as Mohamed Salah's strike looped in via a huge deflection off Simon Kjaer, but then Firmino, presented with the chance to give the home side breathing space, struck a penalty against the post.
Georginio Wijnaldum came close with a low drive in the second half but the English side were punished for not adding to their advantage when they conceded with 18 minutes remaining.
Luis Muriel could have won it for Sevilla in injury time, but shot wide from inside the box, before Reds defender Joe Gomez was dismissed for a second bookable offence.
Midfielder Philippe Coutinho, who requested a transfer in the summer, made his first appearance of the season for Liverpool, coming on for Emre Can on 75 minutes.
In the other Group F game, Maribor came from behind with five minutes remaining to draw 1-1 against Spartak Moscow.
Defensive fragility costs Reds
Liverpool have conceded eight goals in three league games so far this season, and their problems at the back were in evidence again at Anfield.
Manager Jurgen Klopp, a Champions League runner-up in charge of Borussia Dortmund in 2013, did not manage to address the side's defensive issues in the summer, missing out on key target Virgil van Dijk from Southampton.
The lack of understanding among the backline is further complicated by Klopp's apparent indecision about his number one goalkeeper, alternating between Simon Mignolet and Loris Karius for the third straight game, with the German being handed a start this time.
Sevilla's opener came as their opponents failed to close them down on the edge of the box and the ball was played out wide to Sergio Escudero.The full-back drilled a low ball into the box, which Lovren made a hash of clearing, allowing Frenchman Ben Yedder an easy finish at the far post.
For their second, Liverpool were slow to react to a quick throw as former Blackburn midfielder Steven N'Zonzi - an imposing presence for the away side - slipped the ball to Correa, who broke through Liverpool's central defence and stroked in.
To make matters worse, full-back Gomez will miss the next group game through suspension after being sent off in the 94th minute.
Coutinho welcomed back
Before the first round of games, an analytical study by statisticians Gracenote Sports suggested Liverpool had an 81% chance of progressing to the knockout stages - the highest of the six British teams in the competition.
There is work to do now, however, as they failed to hold on to a lead and became the only English side on match day one not to win, amid comfortable victories for Chelsea, Manchester United, Manchester City and Tottenham.
It could have been worse, too, after they fell behind early at Anfield, but the attacking potency of Firmino - four goals this season already - Salah (also four) and Mane (three), often proved too much for the La Liga side as the trio demonstrated their intelligence and speed of play to put the home side ahead and threaten to dominate the game.
But Firmino's first-half penalty miss - awarded after a tug by visiting captain Nicolas Pareja on Mane - was crucial.With it went the prospect of a two-goal cushion, and back came the old defensive doubts.
Midfielder Coutinho returned to action for the first time since the club rejected three bids from Barcelona in the summer, and the Brazilian received a raucous ovation from the crowd, although he failed to make an impact in his 15 minutes on the pitch.
Liverpool host Burnley in the Premier League on Saturday (kick-off 15:00 BST), while Sevilla travel to Girona in La Liga on Sunday (kick-off 15:15 BST).
More to follow....
- ^ 2016 Europa League final, (www.bbc.co.uk)
- ^ analytical study by statisticians Gracenote Sports (www.bbc.co.uk)