Fousseni Diawara scored a late winner as Ajaccio registered just their second win of the season against slumping Rennes.
Diawara converted Johan Cavalli's cross with just seven minutes to go to condemn the visitors to their fifth successive defeat in all competitions.
The win moves Ajaccio on to 12 points, six adrift of safety.
Rennes started well with Jean Kana Biyik glancing a header narrowly over, but then had an almighty stroke of luck after 12 minutes.
Cavalli released Richard Socrier whose header fell perfectly for Benjamin Andre. Andre was left with just Benoit Costil to beat but was hauled down by the Rennes goalkeeper. The home side's shouts for a penalty fell on deaf ears, though, as the referee waved play on.
John Boye shot wide as Rennes came close again after 20 minutes but then only a fine save from Costil denied Cavalli at the other end.
Ajaccio wasted a wonderful chance to go ahead eight minutes into the second half when Cavalli's cross found Mehdi Mostefa but he shot wastefully over the bar.
Rennes were starting to get a grip in midfield and they missed three opportunities to grab the all-important opener.
Youssouf Hadji's curler drifted over, Ajaccio goalkeeper Guillermo Ochoa blocked Jonathan Pitroipa's drive and then produced an even better save to keep out Tongo Hamed Doumbia's top-corner-bound effort.
And Rennes were then caught out by the sucker punch when Cavalli set up Diawara for the winner late on.
