PSG seemed on the way after Neymar set up Dani Alves' superb volley from the edge of the penalty area from a corner and then coolly lobbed the goalkeeper after being put clean through by Angel Di Maria's pass in the 22nd minute.
Rennes striker M'Baye Niang hit the post in the 38th after expertly turning center half Marquinhos and curling a shot past goalkeeper Alphonse Areola. Persistence paid off for Rennes when, two minutes later, a cross from the right forced Presnel Kimpembe into a rash attempt to clear the ball and he, instead, diverted past Areola for an own goal.
PSG's chronic weakness on set-pieces, glaringly exposed in a 5-1 defeat at Lille and a 3-2 defeat at Nantes, again resurfaced when Edson Mexer profited from passive marking to head in Clement Grenier's corner in the 66th.
Neymar set up Kylian Mbappe in the first half of extra time but the prolific France striker hit the post, rather than passing to the unmarked Edinson Cavani, who reacted angrily. Mbappe was sent off just before the end of injury time for a cynical lunge on Damien Da Silva just below his knee.
President Emmanuel Macron, who is a keen fan of PSG's bitter rival Marseille, shook hands and spoke with players from both sides as they were presented to him before the game. Neymar was returning to the starting lineup for the first time since breaking his right foot in January. The Brazil star went on as a substitute against Monaco last Sunday, the day PSG won the league title for the eighth time in its history.
Neymar was shown a yellow card late in the first half for making a late challenge on a Rennes defender. On Friday, Lyon won at Bordeaux 3-2 to pressure Lille in the race to finish second and automatically qualify for next season's Champions League.
Lille is three points ahead of third-placed Lyon and is at home to Nimes on Sunday. Also, fourth-placed Saint-Etienne faces Toulouse, and fifth-place Marseille plays Nantes.
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