Messi also set up the third goal for Lucas Digne with a brilliant piece of skill as Barca fought off thunder and lightning at a soaking wet Camp Nou to make it three wins from three in Europe this season.

Dimitrios Nikolaou opened the scoring with an own goal -- the fifth Barca have benefitted from this season -- before Gerard Pique was sent off for a second booking on the stroke of half-time for batting the ball into the Olympiakos net with his hand.

Nikolaou headed home a late consolation for the Greek side, who remain pointless in this season's Champions League.

"I hope he reaches 200 goals this season!" Barca coach Ernesto Valverde said when asked about another Messi landmark.

"His numbers validate what we see every day, he's beating statistics that seemed impossible. He's been doing it for a long time now, constantly breaking down barriers."

Messi caught the eye in the first half when he took a pill from his sock in the 10th minute, with Catalunya Radio reporting that the Argentine international was taking a glucose tablet.

"I don't know if he took a glucose tablet," said a bemused Valverde. "If it made him score a goal, then he should take a load of them... But I do know the players do take glucose."

One downside to the win was the dismissal of Pique, who will miss the return game in Athens in two weeks after picking up two yellow cards.

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The second booking came when he slapped the ball into the back of the net with his hand, but Valverde played down the incident as "an instinctive gesture."

Barca are now on an 11-game unbeaten run, which includes 10 wins, and it was put to Valverde that the club's supporters have already fallen in love with him in the 100 days since he took charge.

"I don't know if the fans are enamoured with me just yet," he responded. "It's true the results are good. We are top of La Liga, top of our Champions League group... But tomorrow's another day, Saturday's another game. The dynamic can change, football's fanciful like that.

"In August, after the Super Cup [defeat to Real Madrid], we were no good, now we are... So we are going to cross our fingers and keep working hard.

"We are happy today because it's not easy to win in the Champions League. There are no easy games, as we are seeing with other teams.

"Olympiakos came to defend close to their area and to surprise us on the counter-attack. We then had the set back of the red card and we couldn't keep so high up the pitch. When that happens, the rhythm changes.

"But we are satisfied because we were able to open up the game in the second half despite having a man less."