Barcelona travelled to Athens top of Group D and in search of three points to put one foot in the next round, but were met by a hostile crowd and an industrious performance from the hosts to deny them the pleasure.
Ernesto Valverde returned to former club Olympiakos in Barcelona's first-ever visit to the Karaiskakis Stadium. While the reception for the Spanish manager was exceptional, and intimidating atmosphere awaited his players once the game got underway.
The Catalan side were quick to get on the ball and pin the home side inside their own box. The Greeks showed a phenomenal ability to withstand pressure in the early minutes as Barcelona drove forward in search of an opener, with the home crowd zealously helping the cause.
Semedo was the first to fire a warning shot, testing Proto with a shot across goal from the edge of the area which only prompted Olympiakos to sit further back and attempt to repel Barcelona from the back.
With the visitors hogging all the possession, chances came relatively scarce. After 20 minutes, Messi scuffed an effort at goal which Proto just about managed to palm away for a corner, and minutes later Luis Suarez flashed his finish wide after Denis Suarez threaded a delightfully weighted ball through to the striker.
Just when it looked like the hosts were in for a long night, the team seemed to take a step forward. Backed by the ever-chanting home fans, Olympiakos showed more intent as they broke upfield. As pressure from Olympiakos increased, Barcelona began to look more pedestrian with missplaced passes and generally predictable moves that failed to challenge the Greek back line.
Lionel Messi was his usual self for Barcelona and was unlucky not to find the back of the net on a handful of occasions, while Luis Suarez looked slightly off pace, the Uruguayan not even managing to muster the energy he offered the team last season.
The Uruguayan brought the game to life towards the final 20 minutes as he took advantage of the hosts' weariness at the back. With ten minutes to go, Messi floated a superb ball from his own half and Suarez ran in behind the defence to loop the ball over the keeper and hit the top of the crossbar.
The Argentine never stopped trying, and tested Proto at the very end with a free-kick and a firmly struck shot from 18 yards which were well dealt with by the keeper. Barcelona tried their luck right up until the final whistle.
At first glance opponents that Barcelona looked to ease past, Olympiakos proved tough competitors and showed their resilience from start to finish to deny the Catalans their prefect record in the competition.