Olympiakos stunned host Siena with an 82-75 win on Wednesday night, taking a 1-0 lead in the best-of-five series that leads to the Euroleague Final Four.
With the second game scheduled on Friday, also in Italy, and two games to follow in Piraeus next week, if necessary, the Reds have now gained the home advantage in the series.
This crucial victory for the Piraeus club came after a remarkable performance in the last quarter of the game, where the Reds scored a partial 24-7 score to overturn a 10-point deficit (at 68-58) and win by seven in the end.
Olympiakos also had to overcome a hostile crowd that showered the players with projectiles, despite the fact that Vassilis Spanoulis and Richard Dorsey were fouled out.
Siena made the most of Olympiakos's defensive problems in the first half to lead by 48-42 at half-time, but as the game proceeded the Greek defense tightened up and the hosts grew so frustrated that they could not even score from free throws.
Giorgos Printezis was the top scorer for Olympiakos with 20 points, with Spanoulis and Kyle Hines adding 15 apiece.