French leader Marseille's perfect start to the season came to an emphatic end on Sunday, when it lost 4-1 to Valenciennes with goalkeeper Steve Mandanda gifting one of the goals by making a huge blunder.
Marseille's setback, and Lyon's surprise 2-0 home defeat to Bordeaux later Sunday, opened the door for second-place Paris Saint-Germain, which will move into first place on goal difference if it beats Marseille in next weekend's top-of-the table clash.
Lyon is third, one point behind PSG, while Bordeaux maintained its unbeaten start to the season as it climbed into fifth place thanks to second-half goals from left back Benoit Tremoulinas and striker Cheick Diabate
At Valenciennes, Gael Danic and Anthony Le Tallec put the hosts 2-0 up after 35 minutes, and Mandada then rolled the ball out straight to Foued Kadir for the Algeria midfielder to make it 3-0 to the hosts.
Le Tallec scored his second of the game midway through the second half, before Marseille substitute Jordan Ayew pulled a goal back in injury time.
Marseille won its opening six matches and had only conceded one goal prior to Sunday's thrashing.
"It's a heavy defeat, when we went in at halftime 3-0 down the game was over for us," Marseille coach Elie Baup said. "After their first goal we had some chances, we could have got back to 1-1, but we couldn't do it. When you lose like that, it means that very little went right."
Baup refused to criticize Mandanda, who had been in good form prior to the game.
"He's a great professional, I don't want to single anyone out," Baup said. "The whole team didn't function."
Marseille lined up with a three-man wall after giving away a free kick in the 15th minute, and Danic curled the ball superbly into the top right corner.
Poor defending led to the second goal, as right back Rod Fanni hesitated as he tried to clear the ball and Danic took it off him before finding Le Tallec with a pass.
After scoring the third goal in the 39th, Kadir set up the fourth, breaking down the left and crossing the ball for Le Tallec, who beat Mandanda with a downward header.
"We had to lose one day, but not like this. We weren't good enough, it's as simple as that," Marseille midfielder Benoit Cheyrou said. "We have to be critical of ourselves, so that we can get back on track."
With France's World Cup qualifier away to world and European champion Spain a little over two weeks away, Mandanda's performance will be of concern to coach Didier Deschamps, with his first-choice goalkeeper Hugo Lloris stuck on the bench at Tottenham and lacking match practice.
Marseille has a big week coming up with a home game against AEL Limassol in the Europa League on Thursday, followed by a home match against bitter rival PSG on Sunday.
"We have to put this game to one side and look ahead," Baup said. "We will have to pull ourselves together, analyze what went wrong in the cold light of day and get back to how we were playing before."
Lyon, meanwhile, dominated for almost the whole match before being caught out by two counterattacks as it surrendered its unbeaten record.
As Bordeaux's five-man defence sat back and soaked up pressure, Lyon had its first shot on target in the 32nd when goalie Cedric Carrasso made a brilliant save low to his right to keep out midfielder Clement Grenier's shot.
No team scored first against Bordeaux this season.
"It's hard to score against them, you have to look for spaces, look for width and play one-twos," Lyon coach Remi Garde said. "You need to try and get the extra man against them, but it takes patience."
Lopez sliced his shot wide right at the start of the second half after running onto Steed Malbranque's excellent pass from midfield.
Carrasso then made excellent saves to deny striker Bafetimbi Gomis and Grenier, before Bordeaux stunned the home crowd in the 65th when Tremoulinas ran onto a pass from Ludovic Obraniak and his shot deflected off centre back Dejan Lovren.
Carrasso foiled Gomis again moments later, and from the resulting corner Alexandre Lacazette's header went in but the goal was disallowed after the linesman ruled that the ball had gone out of play when the corner was taken. Television replays showed the ball was entirely in play.
A mix-up in defence saw Lovren struggle to clear his lines and Diabate beat him to the ball to head it over goalie Remi Vercoutre in the 82nd.
Also, Reims extended its unbeaten run in the league to five matches after battling to a 0-0 draw at Saint-Etienne. Reims is in eighth place, one spot behind Valenciennes.
PSG beat Toulouse 2-0 at home on Saturday.