Having slipped to second place last weekend, reigning French champions Marseille risk seriously compromising their title chances when they visit Lyon on Sunday.
Marseille's surprise 1-1 draw at home to Auxerre allowed Lille to return to the summit, and Rudi Garcia's team could go provisionally four points clear with victory at third-bottom Nancy on Saturday.
Lyon's Stade Gerland is a challenging venue at the best of times but the home side are in particularly dire need of points after losing 2-0 at Toulouse.
The defeat allowed Paris Saint-Germain to draw level on points with Lyon in third place - the final Champions League qualifying berth - but Marseille coach Didier Deschamps says his side will not sit back and allow Lyon to attack them.
"Going to Lyon to defend is not the best solution," Deschamps told Marseille's in-house television station, OMtv.
"We will certainly have to defend well against an opponent that attacks well but we must also create problems for them by keeping a balance."
"And I prefer my team to have the ball rather than running after it."
Midfielders Benoit Cheyrou and Lucho Gonzalez are expected to make the Marseille squad after sustaining injuries against Auxerre, while forward Loic Remy returns from suspension following his red card at Montpellier.
Sunday's game will bring Lucho up against his Argentine compatriot, and former Porto teammate, Lisandro Lopez, who has upbraided his own teammates over their dire performance at Toulouse.
"It was a disaster on every level," said the striker.
"If we want to take something positive from it, it's that everyone makes mistakes and we cannot make errors like that in our remaining matches."
Lyon will be without left-back Aly Cissokho and winger Michel Bastos, after both were sent off in the second half at Toulouse, while centre-backs Jeremy Toulalan and Cris are both doubtful with injuries.
PSG can steal a march on Lyon with victory at Monaco on Saturday, although the side from the principality have lost just once in their last five matches to clamber out of the relegation zone.
With eight teams separated by just four points in the bottom half of the table, Ligue 1 appears destined for a nail-biting relegation scrap.
Bottom club Arles-Avignon have already been relegated and Lens, nine points from safety, look guaranteed to join them, but even 11th-placed Toulouse are still to break free from the mire at the table's foot.
Nancy slipped into the drop zone after losing 1-0 at Sochaux last weekend and on Saturday they face a Lille team with the championship bit firmly between their teeth after their 5-0 demolition of Arles.
"If we play our football, I'm sure that we'll at least qualifying for the Champions League and we can win the championship," said Lille centre-back David Rozehnal.
"We're the leaders. We're in front of Marseille, who have a difficult match against Lyon. For us, it's better to be in front."
In the race for sixth place, and Europa League football next season, sixth-placed Lorient host Toulouse, with Bordeaux - two points behind Lorient in eighth place - at home to seventh-placed Sochaux.
Saturday (1700GMT unless otherwise stated):
Arles-Avignon v Saint-Etienne, Auxerre v Montpellier, Bordeaux v Sochaux, Brest v Nice, Caen v Lens, Lorient v Toulouse, Monaco v Paris Saint-Germain, Nancy v Lille (1900GMT)
Valenciennes v Rennes (1500GMT), Lyon v Marseille (1900GMT)