The northen side, who are on 60 points with six games left and have failed to win in their last three league matches, are under pressure from Olympique Marseille, who play their game in hand on Wednesday.
Lille, leading the standings since December, were dominated throughout but opened the scoring when Mathieu Debuchy volleyed home from a narrow angle three minutes before the interval.
Lorient increased the pressure and were rewarded in the 68th minute when French international striker Kevin Gameiro's volley beat Mickael Landreau in a lively game in Brittany.
PSG, who went ahead through Mathieu Bodmer but conceded goals by Larsene Toure and Bruno Grougi before the break, salvaged a point thanks to Guillaume Hoarau's 87th-minute strike and they remain fourth on 53 points.
Antoine Kombouare's side trail Lille by seven points.
PSG were hoping to follow up on last week's win over third-placed Olympique Lyon.
Bodmer opened the scoring with a low 20-metre drive but the game turned the hosts' way when Apoula Edel replaced injured keeper Gregory Coupet in the 34th minute.
Edel fouled Toure in the box to gift Brest a penalty. The keeper parried Grougi's attempt, only for Toure to poke the ball into the net eight minutes from the break.
On the stroke of half-time, Toure's cross found Grougi in the box for the midfielder to score through Edel's legs.
PSG, however, equalised three minutes from time when French international striker Hoarau netted from close range after the ball took a deflection off Brest defender Ahmed Kantari's hand.
RC Lens's hopes of staying in the top flight were dashed with a 1-1 draw at AJ Auxerre that left the northerners second from bottom and eight points from safety.
