PSG spent millions in the close-season thanks to new Qatari owners but lost their opener at home to Lorient last weekend and only picked up a point in Brittany despite striker Kevin Gameiro scoring his first goal for the club after 73 minutes.
"When you lead 1-0 with two minutes left you have to be more solid than that. You can't be naive like we have been with the goal," under-pressure PSG coach Antoine Kombouare told reporters.
Olympique Lyon, winners at Nice in their first game under new coach Remi Garde, dominated again but had to settle for a 1-1 draw with promoted visitors Ajaccio thanks to Lisandro Lopez's superb headed equaliser eight minutes from time.
Lyon face Russia's Rubin Kazan in their Champions League play-off first leg on Tuesday.
Gameiro's ex-club Lorient were pegged back late on to draw 1-1 at home to Girondins Bordeaux while Toulouse made it two wins out of two with a 2-0 victory over promoted Dijon and Caen joined them on six points after a 2-1 success at Sochaux.
Saint-Etienne are also riding high following defender Sylvain Marchal's late goal in a 1-0 home victory over Nancy.
PSG's Kombouare, whose job has been linked with Italian coach Carlo Ancelotti after just one defeat given the high expectations racing through the French capital, decided against giving record signing Javier Pastore his debut.
The Argentine, signed from Palermo in a reported French record 42 million euro deal, is not fully fit so the playmaker duties were left to Jeremy Menez.
The former AS Roma man set up Gameiro for the cool finish but Rennes, who beat Dijon 5-1 last weekend, played their part in an enjoyable contest and got their reward when Burkina Faso's Pitroipa got round the back of the defence to score.
Valenciennes picked up their first point after being held goalless by Stade Brest who claimed a second draw.
