An 89th minute goal from Nancy winger Yohan Mollo handed PSG a 2-1 defeat - their first league loss under new boss Carlo Ancelotti - and saw them slip behind Montpellier at the top of Ligue 1.
Starting the day level on points with Montpellier at the top, Carlo Ancelotti`s men could have moved three points clear with a win, with their rivals not in action until Sunday.
Nancy had other ideas, though, taking the lead through Bakaye Traore and then winning it with Mollo's 89th-minute strike after Mohamed Sissoko had levelled.
Kamel Chafni and Dennis Oliech scored as bottom side Auxerre improved their chances of escaping relegation with a vital 2-0 victory over mid-table Valenciennes.
Auxerre opened the scoring after 25 minutes as Oliech and Omar Kossoko combined to find Chafni on the right. Chafni beat David Ducourtioux and Gil before firing past Nicolas Penneteau at his near post.
And the game was wrapped up as Roy Contout picked out Oliech and he beat both Mody Traore and Gil before hitting a shot which went in off the post.
Nice climbed out of the relegation zone by coming from behind to secure a dramatic 3-2 victory over St Etienne at the Stade Geoffroy-Guichard.
Les Verts, looking to re-ignite their hopes of European football next season, took the lead in the 16th minute through Max Gradel and were 2-1 up at the break thanks to Sylvain Marchal's 42nd-minute header after Anthony Mounier had equalised.
But the struggling visitors stunned their hosts after the interval, Mounier's second of the night levelling things up before Francois Clerc's winner two minutes from time.
Substitute Thomas Guerbert scored with his first touch to snatch a dramatic point for Dijon, 1-1 away to Bordeaux.
Benoit Tremoulinas opened the scoring from the penalty spot in the 72nd minute, but Guerbert equalised a minute from time to lift the visitors four points above the relegation zone.
Sochaux's bid to escape relegation received another big boost as they struck twice in the last 14 minutes to claim a comeback 2-1 win over fellow strugglers Brest.
Brest had looked set to claim a vital victory after taking an early lead through Bruno Grougi, but Modibo Maiga drew Sochaux level in the 76th minute and substitute Slovan Privat netted what proved to be the winner five minutes later.
The victory was Sochaux's third in their last four games and lifted them out of the bottom two and level on 30 points with Brest, who have now lost four on the trot.
Lorient remained in a precarious position towards the bottom of the table after falling to a disappointing 1-0 home defeat against Evian.
With just four points separating themselves from the drop zone prior to kick-off, Christian Gourcuff's men knew a three-point haul would provide them with a much-needed, late-season boost heading into the final fixtures.
It was not to be, though, with Brice Dja Djedje's 19th-minute goal enough to keep Les Croix de Savoie tucked away safely in mid-table.