Benoit Pedretti made a dramatic entrance for Lille as he scored a winner in the dying seconds after coming on as an 87th-minute substitute at St Etienne.
Aurelien Chedjou put Les Dogues, who finished third in Ligue 1 last season, on the front foot with a 58th-minute goal but Romain Hamouma levelled in the 74th minute for Les Verts.
The match looked set to end in a draw until Pedretti's crucial intervention.
A chance came for St Etienne after just two minutes when Max Gradel took his chance from 25 yards in the middle of the pitch but, although his shot was on target, Lille's Mickael Landreau blocked it.
Bakary Sako tried his luck from 25 yards after 32 minutes with a nice half-volley that Landreau managed to keep out.
Lille responded as Dimitri Payet crossed for Nolan Roux to shoot but Stephane Ruffier made an excellent reflex save on the line.
The action picked up in the second half when Chedjou scored for Lille, getting the ball past Ruffier in the 57th minute of the game with a header from only six yards.
But an equaliser came in the 73rd minute when Hamouma scored after Sako crossed the ball in.
Lille came back again though and Pedretti grabbed the winner. Payet crossed from the left and Ruffier could only push the ball toward Pedretti, who volleyed brutally with his right foot to send the ball into the top corner of the goal.