Atletico Madrid slipped to a surprise 3-2 defeat to Olympiakos on Tuesday in a disappointing return to the Champions League for last season's finalist.
Atletico fell behind in the 13th minute of the Group A match when defender Arthur Masuaku aimed a left-footed drive past goalkeeper Jan Oblak, who was making his debut for the Spanish club.
Striker Kostas Mitroglou unselfishly left the ball to Imbrahim Afellay who made it 2-0 in the 31st. But Mario Mandzukic pulled a goal back before the break, left unmarked to head in a cross from Cristian Ansaldi.
Mitroglou scored with a stylish move that proved to be the winner in the 73rd, turning on the ball to beat Oblak, before Antoine Griezmann finally found the net in the 86th for Atletico. Griezmann was twice denied by Olympiakos goalkeeper Roberto - who joined Atletico as a schoolboy and was loaned out by the club last year to Greece.
''This is a huge win for us,'' said Olympiakos captain Giannis Mantiatis, who recovered from an ankle injury to make the starting lineup. ''We had to sweat the whole match. These were very valuable points we got tonight.''
Atletico coach Diego Simeone watched from the stands at Karaiskaki stadium, as he served a one-match European ban.
''We threatened throughout the game, but Olympiakos was very tough and they got a valuable win,'' Simeone said.
Simeone defended his choice to replace first-choice goalkeeper Miguel Angel Moya, arguing that rotation was needed to cope with the domestic and European workload.
''What decided the game was that they were more effective with their chances that we were,'' he said.
''Obviously this was a very negative result for us. Clearly Olympiakos is in a good position. What we feel now is a sense of responsibility.''
Olympiakos coach Michel Gonzalez said his team ''successfully disrupted'' Atletico.
''To beat a team like Atletico, you have to do a lot of things right ... We are a team like (Atletico) and do well against teams that appear to be of higher quality,'' the Spanish manager said.
''Now we have an advantage, to be sure,'' he said. ''All I can say is that we are very happy - much happier than we were two hours ago.''
Juventus beat Malmo 2-0 in the group's other game.
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