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Lyon saw their French title hopes suffer a potentially fatal blow on Sunday as they went down 2-1 at home to relegation-threatened Sochaux.
The loss left Lyon in third place, eight points adrift of leaders Paris Saint Germain, who now look odds-on to lift their first domestic crown since 1994.
Marseille are a point better off than Lyon, who have dropped out of the top two for the first time since November.
Marseille shrugged off having their supporters banned from attending their clash with Nice, which brought a 1-0 success to boost the 2003 European champions' chances of qualifying for next season's Champions League.
"Seeing off a direct rival here in Nice was not easy - but necessary," said Marseille coach Elie Baup.
French international striker Andre-Pierre Gignac scored the only goal of the Marseille game, striking in the 15th minute with his 10th goal of the season after being set up by Mathieu Valbuena.
The match had had a lively introduction as Marseille's combative sporting director Jose Anigo had a blazing row with Nice director-general Julien Fournier, who used to be part of the Marseille board.
The players also sparked controversy as in the pre-match warm-up they wore shirts emblazoned with 'Marseille supporters present' in protest at the ban on their fans being allowed to come to the match.
Paris Saint Germain beat defending champions Montpellier 1-0 on Friday.
Saint Etienne are in fourth spot after a 2-2 draw with relegation-threatened Troyes on Saturday.
The top three clubs qualify for the Champions League, the top two for the group stages and the third for the final qualifying round.
Lyon were undone at the Stade Gerland when, after 45 minutes of sterile fare, the hosts went behind five minutes into the second half to a header from Giovanni Sio, who bulleted in his effort following a cross from Ryad Boudebouz.
Lyon were level on 68 minutes from the penalty spot after David Sauget fouled Alexandre Lacazette.
France international striker Bafetimbi Gomis just squeezed the ball past Simon Pouplin in the Sochaux goal.
But instead of pressing for the winner, Lyon again lost their way and fell victim to a sucker punch as Cedric Bakambu swept home two minutes from the whistle to take Sochaux out of the drop zone at the expense of Brest.
Also Sunday, Lille won 2-1 at Brest to leave the Breton side third bottom but Lille in fifth a point adrift of Saint Etienne with Ivory Coast star Salomon Maoulida and Nolan Roux on target.