Former Nottingham Forest player Patjim Kasami scored the only goal of the game as Olympiacos took a 1-0 victory over the Premier League side.
Kasami, who was loaned out to Forest by the Greek side during the 2016-17 season, headed home from a corner in the first-half.
Forest made a host of changes during the match in a bid to mount a fight-back, but Steve Cooper's side suffered their second defeat in Greece - after Tuesday's 3-2 loss to Atromitos.
More to follow...
MATCH FACTS
Nottingham Forest: Henderson; Aurier, Boly, McKenna, Lodi; Colback, Mangala, Yates; Gibbs-White, Lingard, Johnson
Olympiakos: Tzolakis; Reabciuk, Retsos, Sokratis, Vrousai; M'Vila, Kasami, Camara; Pep Biel; Fortounis, El Arabi
Goal: Kasami 40