Lyon have 57 points from 31 matches, one ahead of Bordeaux who dropped to third but have two games in hand.
Bordeaux also trail Lyon 3-1 going into their midweek Champions League quarter-final second leg.
"We didn't get a lot of positive signals before Wednesday's game. We were mediocre at too many aspects of the game," Bordeaux coach Laurent Blanc told reporters after his side suffered their third defeat in a row.
"Too many guys were below par."
"Hats off to my players who were very strong," said Lyon coach Claude Puel. "But my joy has already gone and I'm already focused on Wednesday's game."
Rennes opened the scoring in the 14th minute when Ghana striker Asamoah Gyan capitalised after Jimmy Briand's shot bounced back off a post.
Fernando Cavenaghi tapped in Bordeaux's equaliser from an Alou Diarra header before Dia, who grabbed a hat-trick last week, gained maximum points for Nancy when he scored after running from his own half in the 71st minute.
