Barcelona must to wait to confirm a place in the Champions League last 16 after dropping points for the first time in Group D in an absorbing 0-0 draw at Olympiakos Piraeus on Tuesday.
On his return to his former club, Barca coach Ernesto Valverde watched his in-form side dominate for long spells and create some great chances, but the determined home team held on for their first point as they bid for a Europa League spot.
Olympiakos goalkeeper Silvio Proto was a prominent figure in a frenetic and surprisingly open match at the Georgios Karaiskakis stadium, coming to the rescue to deny Lionel Messi and Luis Suarez on several occasions.
Barcelona top the section with 10 points, having extended their unbeaten run in all competitions to 15 matches with 13 wins and two draws, ahead of Juventus on seven points after their 1-1 draw at Sporting who have four.
Takis Lemonis' Olympiakos team, who are also struggling in the Greek SuperLeague, finally opened their account as Barca were held to a goalless draw for the first time since April.
Valverde, who won three league titles as Olympiakos coach in two spells at the Piraeus club between 2008 and 2012, was given a rousing welcome by the home fans who held up a giant banner which read “Ernesto thanks for everything”.
“I was very moved by the whole experience of coming back here tonight,” said Valverde. “I love the fans here and they appreciate my work; it was amazing to see such a reception for an opposing coach – it speaks volumes for them really.”
“Olympiakos is a very strong team and with the help of their fans they have achieved a great result and we congratulate them for that. I would love to see them qualify to the next stage but there is a long way to go yet.”
Barca started at breakneck speed with right back Nelson Semedo racing clear and forcing a fine low save from Proto.
Olympiakos, with midfielder Kostas Fortounis in a lone striking role, were pushed back in the early stages but the Greece international looked lively when he broke forward.
Barca came close to scoring in the 20th minute when Messi struck a shot that was parried clumsily for a corner by Proto.
His strike partner Suarez missed Barca’s best chance of the half three minutes later, sending a low shot into the side-netting with the outside of his right boot from a tight angle.
Messi’s first real run at the defence came in the 28th when the Argentine raced through a flurry of bodies into the box before his shot was saved by Proto's outstretched left boot.
Messi then had a shot blocked before seeing his curling free-kick, won by Suarez, beaten away by Proto.
Fortounis should have done much better from Alaixys Romao’s lobbed pass on the counter but volleyed straight at Marc-Andre ter Stegan who gathered it comfortably.
The visitors forged their best chance just after the hour but Messi uncharacteristically scuffed a poor first-time effort wide after a neat one-two with Suarez.
Barca exerted more pressure as the second half wore on but had little to show for their efforts apart from a couple of blocked and wayward shots by a below-par Suarez, who also lobbed an effort on to the top of the bar with 10 minutes left.
Proto made two more stops to thwart Messi, one from a free-kick and another from a powerful low shot, to ensure Olympiakos secured a creditable draw against their illustrious opponents.