Zlatan Ibrahimovic scored his eighth goal of this season's Champions League campaign as he put Paris Saint-Germain in front against Olympiacos at the interval.
The Swedish forward, making his 100th appearance in the competition, edged into space to latch on to a Gregory van der Wiel cross from the right and tapped home past Roberto in the visitors' goal with only seven minutes on the clock.
Kostas Mitroglou tried to bring Olympiacos back on level terms soon after as he met Joel Campbell's floating ball with a first-time effort, which drew a good save out of Salvatore Sirigu.
The Greek forward threatened again midway through the first period when his low near-post effort had to be palmed away by the PSG keeper.
However, the hosts dominated possession and almost doubled their advantage before half time when Edinson Cavani's thunderous effort was stopped by Roberto.
A win will guarantee Les Parisiens top spot in Group C, but Olympiacos know that three points would secure themselves passage through to the last 16.
