Nice defeat leaves Lille hanging on to Champions spot
A supporter of Lille holds the club's scarf before the French L1 football match between Paris Saint-Germain (PSG) and Lille (LOSC) on December 22, 2013 at the Parc de Princes stadium in Paris - by Lionel Bonaventure
Saint-Etienne's 3-0 success over struggling Valenciennes on Saturday put the pressure on Lille but the 2011 champions failed to respond.
Former Paris Saint-Germain forward Mathieu Bodmer scored the only goal with a cute back-heel in first half stoppage time, although there was a suspicion of offside.
At the turn of the year, Lille were challenging the top two, PSG and Monaco, but they have now dropped to 13 points off the summit after earning just two points from their last five matches.
Things could even get worse if fifth placed Marseille and Lyon in sixth both win later on Sunday.
If that happened there would be three teams within four points of the third-placed northerners.
The defeat could have been worse for Lille but for Rio Mavuba clearing a Dario Cvitanich header off the line and Idrissa Gueye getting away with what appeared a blatant handball inside the Lille box.