Lens missed the chance to move top of Ligue 1 and put pressure on Paris Saint-Germain as they slipped to a 2-1 loss at Lorient on Saturday.
Pierre Sage's side were aiming to go two points clear of PSG at the summit, with the reigning champions not playing this weekend so they can rest for Tuesday's Champions League last-16 second leg at Chelsea.
But Lens, targeting a second Ligue 1 title and first since 1998, fell to a costly defeat and have now won only one of their last four league matches.
Bamba Dieng put Lorient into an 18th-minute lead, bundling in from close range at the second time of asking after Lens failed to deal with a corner.
The visitors hit back, though, shortly after half-time, as Odsonne Edouard collected the ball inside the Lorient half, carried it to the edge of the box and unleashed a low shot into the bottom corner to score his 10th league goal of the season.
Lens had the momentum and went close to taking the lead shortly before the hour mark when Florian Thauvin headed a corner narrowly over.
But Lorient regained the advantage in the 65th minute, as Aiyegun Tosin stabbed the ball in from almost on the goal-line after just about controlling Arsene Kouassi's cross.
Lens piled on the pressure late on with a series of set-pieces but could not force another equaliser.
Lorient climbed to eighth in the table, just four points behind sixth-placed Lille, who visit fifth-placed Rennes on Sunday, in the race for European qualification.