Lens suffered a potentially fatal blow to their unlikely Ligue 1 title bid with a 3-0 loss away to northern derby rivals Lille on Saturday.
Pierre Sage's team have surprisingly been Paris Saint-Germain's closest challengers in France this season but they were swept aside by an in-form Lille.
Hakon Haraldsson turned in Matias Fernandez-Pardo's clever pass just before halftime at the Stade Pierre-Mauroy.
Felix Correia pounced on a stray back-pass from Matthieu Udol to score a second for Lille five minutes after the break.
Fernandez-Pardo sealed a fifth win in six games for Lille from the penalty spot after a handball by Ismaelo Ganiou.
"You can lose but not in the way we did in a game like this. Now we have a duty to react," Sage told broadcaster Ligue 1+.
"We didn't turn up. It's as if we were looking at ourselves in the mirror and seeing a different version of us."
Lille climbed up two places to third after extending their unbeaten run to eight matches, with Lens still four points behind PSG and having played a game more than the league leaders, who brushed past Toulouse 3-1 on Friday.
Strasbourg beat Nice 3-1 despite losing Argentina striker Joaquin Panichelli, the league's joint leading goalscorer, to a long-term knee injury suffered while on international duty last week.
Martial Godo, Julio Enciso and Samir El Mourabet all struck in the first half for Strasbourg before Antoine Mendy bagged a late consolation for Nice.
Esteban Lepaul joined Panichelli on 16 goals for the season as he converted a pair of penalties to lift Rennes to a 4-3 win at Brest.
Swiss forward Breel Embolo also scored as Rennes moved up to sixth after inflicting a third straight loss on mid-table Brest.