Qatari-backed PSG remained second on 67 points, two ahead of champions Lille who moved back into contention for automatic Champions League place.
Unheralded Montpellier took a big step toward their first league crown when they won 1-0 at Toulouse on Friday.
"It will be more difficult now to clinch the title but we have to keep on working and believing it's possible," PSG coach Carlo Ancelotti said.
"We also have to watch our backs to seal our Champions League qualification because Lille are only two points behind."
"We are disappointed because we did not deserve to lose. It will be important to react immediately."
In-form Eden Hazard, who has now scored eight goals and set up six others in his last eight games, cancelled out the opener from the penalty spot.
Roux clinched the hosts' victory eight minutes later when he headed home from close range from a Tulio de Melo header after Belgian Hazard initiated a superb passing move.
Pastore and Hazard had their chances to score in the first half but the Argentina playmaker's fine volley hit the woodwork in the 27th minute. Hazard was also denied by the post.
Rennes won their fourth consecutive league match to occupy the first Europa League spot on 57 points and are one ahead of French Cup winners Olympique Lyon who have a game in hand.
Feret gave his side the lead when keeper Guillermo Ochoa deflected a Jires Kembo shot on the woodwork only to see Rennes playmaker head the ball in on the rebound in the 27th minute.
Razak Boukari doubled their advantage with five minutes to go as he bent the ball into the top right corner from outside the box before Yacine Brahimi ran through to seal the victory in stoppage time following a Feret pass.
Ajaccio, a point and a place above the drop zone, were reduced to 10 men after just 11 minutes when Samuel Bouhours was sent off for a harsh tackle on Jonathan Pitroipa. Ilan's 89th minute goal was of no use.
Les Verts are sixth on 56 points while Dijon lie 16th on 34 points, one place above Ajaccio but on the same points.
Auxerre, who have been playing in Ligue 1 since 1980 and featured in the Champions League group stage last season, are three points from safety.
Brest sacked coach Alex Dupont this week after they lost six of the last seven games and stay 18th on 33 points while Sochaux are a spot below and on the same points having lost 3-0 at home to mid-table Girondins Bordeaux as they were reduced to nine men.