Substitute Bafétimbi Gomis struck late to restore AS Saint-Etienne's two-goal advantage as Olympiacos CFP's run of 16 home wins in all competitions this season came to an abrupt end.
First-half strikes from Ilan and Geoffrey Dernis set Alain Perrin's side on their way to victory before Predrag Djordjević pulled a goal back from the penalty spot in the 64th minute. However, Gomis's stoppage-time effort sealed a result which put the French club firmly in the driving seat ahead of the return leg next Thursday.
Boasting a perfect home record this term, Olympiacos began brightly with a Vassilis Torosidis attempt ruled out for offside after four minutes. Yet they went behind in the 12th minute when defender Didier Domi lost possession to Paulo Machado. His pass bounced into the path of Ilan, who slotted into the bottom-right corner from ten metres. The Brazilian might have had a second soon after but missed from an acute angle after Dernis's fierce drive was parried into his path by home keeper Antonis Nikopolidis.
Olympiacos skipper Djordjević also missed a golden chance, nodding over from point-blank range after Luciano Galletti's superb right-wing cross. However, just as the Greek titleholders looked to be in the ascendancy, St-Etienne struck again – the luckless Domi's block from Kevin Mirallas's shot landed at the feet of Dernis, who rifled the ball low past Nikopolidis two minutes before the interval.
The hosts went agonisingly close ten minutes after half-time as Dudu's header was brilliantly tipped over by keeper Jérémie Janot. Olympiacos's endeavour was finally rewarded when Djordjevíć converted coolly from the spot after Christos Patsatzoglou was felled by Cédric Varrault. Dudu came within centimetres of heading his side level with seven minutes left, but Gomis – who had replaced the injured Ilan – had the last say as his long-range strike deflected into the top-left corner off Domi.