Lorient endured a night to forget as Valenciennes took advantage of two red cards with a late flurry of goals.
The visitors actually took an early lead through Benjamin Corgnet but were 2-1 down before Arnaud Le Lan's 18th-minute dismissal, Carlos Sanchez and Anthony Le Tallec doing the damage.
Gael Danic's penalty and a second Lorient red card - Innocent Emeghara failing to live up to his name - put the result beyond any serious doubt and two goals in the final 10 minutes from Vincent Aboubakar, plus another for Le Tallec, inflicted maximum punishment on Christian Gourcuff's side.
Their bright start was therefore forgotten, Alain Traore going close even before Corgnet headed home Kevin Monnet-Paquet's cross in the sixth minute.
That was as good as it got, though, and visiting keeper Fabien Audard had to deny Foued Kadir before Sanchez headed in Danic's 10th-minute corner to equalise.
Le Tallec swept home Gaetan Bong's cross three minutes later and when Le Lan collected a second booking for dissent, Lorient were really up against it.
They reached half-time without further mishap but Wesley Lautoa's foul on Mathieu Dossevi allowed Danic to make it 3-1 from the spot six minutes after the restart.
Monnet-Paquet volleyed wide in a rare Lorient attack but once they lost Emeghara, following a nasty tackle from behind on Carlos Moreno with 15 minutes remaining, they fell apart completely.
Danic set up Aboubakar to score from 20 yards and Le Tallec superbly controlled Dossevi's headed pass on his heel before firing the ball into the top corner for his side's fifth goal.
Aboubakar got ahead of the shell-shocked Audard to cut in Danic's cross two minutes from time and add the final insult.