Wahbi Khazri scored an 85th-minute equaliser for Bastia to deny Lille all three points in Corsica this evening.
The midfielder's late header cancelled out Salomon Kalou's 32nd-minute strike, but Lille still extended their unbeaten Ligue 1 run to 12 matches with a 1-1 draw.
Former Chelsea striker Kalou missed a chance to put the visitors ahead on nine minutes when he dragged a shot wide from 12 yards, and Marvin Martin had earlier tested Mickael Landreau in Lille's bright start.
Bastia were knocking on the door themselves in the opening stages, though, with Gianni Bruno and Francois Modesto heading over the crossbar from successive corners in the 15th minute.
Landreau was called upon again on 22 minutes when he denied Nolan Roux, who was one-on-one with the goalkeeper following Martin's through-ball, while Kalou twice shot wide from distance.
The Ivorian found his radar just after the half-hour mark to give Lille the advantage, applying the finishing touches to a swift counter-attack which also involved Roux to score his 14th of the season.
For a long time it appeared as if though that would be enough for Rene Girard's side to secure a fifth victory in six matches, but Khazri's fifth goal of the campaign earned midtable Bastia a point.
Having missed a couple of opportunities earlier in the second half, the Tunisian international headed in Florian Raspentino's corner as Bastia finally made their threat from set pieces count.
The result means that Lille can no longer catch Monaco in second and are guaranteed to finish third.
