Patrick Vieira's Nice slumped to a fourth defeat in five home league games this season as Morgan Sanson's first-half strike fired Marseille to a 1-0 win in Ligue 1 on Sunday.
It was a laboured performance from Nice, who sit 14th in the table and only two points above the relegation play-off spot, eight points adrift of fourth-placed Marseille.
Vieira may need a good run of results if he is to keep his job long enough to take on former Arsenal and France teammate Thierry Henry when his side visit Monaco on December 8.
For Marseille, all eyes will now be on next Sunday's clash with arch rivals and runaway league leaders Paris Saint-Germain at the Velodrome.
The hosts went into the match having mustered only seven goals in nine league games this season, and Vieira responded by handing Mario Balotelli just his third start of the campaign up front.
The first decent chance at the Allianz Riviera fell to the Italy international, but after latching onto a long ball forward from Dante his well-struck volley was kept out by the legs of Marseille goalkeeper Steve Mandanda.
A clever dummy from Balotelli created space for Allan Saint-Maximin, but his shot was tame and easily gathered by Mandanda.
The away side grabbed the lead in the 42nd minute, though, as full-back Bouna Sarr skipped clear down the right-hand side and crossed for Sanson to smartly turn and fire into the bottom corner.
Nice plugged away in search of an equaliser but never looked like having the quality to break through, with centre-back Dante's looping volley onto the roof of the net the best effort Vieira's men could muster.
Earlier on Sunday, Montpellier moved into third as goals from Gaetan Laborde and Andy Delort sealed a 2-0 win over Bordeaux, while Saint-Etienne drew 1-1 against Rennes.
Henry suffered a miserable return to French football on Saturday as Monaco lost at Strasbourg in his first match as a head coach, while PSG thrashed Amiens to make it 10 successive wins to start the season.