The northerners recovered from a shaky start to prevail with goals by Tulio de Melo and Rio Mavuba and now have 69 points with three games left.
Second-placed and champions Olympique Marseille will play their game in hand at home against Stade Brest on Wednesday.
Paris Saint-Germain's hopes of securing a qualifying spot for the Champions League third qualifying round took a knock when they were held to a 2-2 home draw by relegation-threatened Nancy that left them in fourth place, one point adrift of Olympique Lyon.
At St Etienne's Geoffroy Guichard stadium, the Lille defence were caught snoozing early on as Laurent Battles collected a through ball, stumbled on Mickael Landreau but Emmanuel Riviere, who had followed, poked the ball into an empty net to make it 1-0 after six minutes.
The visitors reacted swiftly and Tulio de Melo levelled with a delicate volley from the spot from an Eden Hazard cross in the 14th minute.
St Etienne were awarded a penalty after the referee harshly ruled that Riviere had been fouled by two Lille defenders but Landreau parried away Bakary Sako's attempt on 18 minutes.
Lille increased the pressure after the break and were rewarded in the 66th minute when midfielder Rio Mavuba unleashed a fierce 30-metre shot that beat Jeremy Janot after taking a sllight deflection off Sylvain Monsoreau.
"We have a goal and we're trying to do everything we can to reach it," Mavuba told French TV channel Canal Plus as he was congratulated by his team-mates.
"(After St Etienne opened the scoring) we kept our cool, we just tried to play some good football and did what we had to do."
PSG, who will face Lille in the French Cup Final on Saturday at the Stade de France, got off to a brilliant start as they opened the scoring thanks to a Mevlut Erding header in the fourth minute.
Midfielder Christophe Jallet, however, was then sent off in the 20th minute after fouling Julien Feret in the box. Landry Nguemo converted the resulting penalty to make it 1-1 but PSG did not give up.
On the stroke of half-time, defender Zoumana Camara put the hosts ahead with a brilliant strike on the turn, only for Youssouf Hadji to equalise with a header in the 69th minute.
Nancy were also reduced to 10 men two minutes later after Reynald Lemaitre was shown the exit for picking a second yellow card.
PSG pushed harder with Nene hitting the woodwork with a curled free-kick but the visitors held on.
Nancy are still in the relegation zone on 39 points, one point from safety.
