PSG slumped to their fourth Ligue 1 defeat of the season as they were beaten 2-1 at Guingamp.
Unai Emery’s side were stunned by second-half goals from Yannis Salibur and Nill de Pauw and, although Edinson Cavani pulled one back in the 80th minute, the reigning champions were left four points adrift of leaders Nice.
PSG almost salvaged a point deep into stoppage time, but Cavani’s looping header came back off the crossbar before Jean-Kevin Augustin had a very late effort cleared off the line.
Guingamp’s second win in seven league games sees them move up to fourth place, seven points behind third-placed PSG, who had already lost at Monaco, Toulouse and Montpellier this season.
Bastia’s resurgence in form continued as they picked up a second win on the bounce, seeing off Rennes 2-1 in Brittany.
The visitors took the lead with a superb counter-attack, as Sadio Diallo broke from deep before feeding Enzo Crivelli, who slotted coolly into the corner.
The lead was doubled shortly after the hour mark, as Gael Danic latched onto a knock-down and slotted home.
Giovanni Sio acrobatically turned in a Kamil Grosicki cross with 20 minutes to play, but the hosts could not grab an equaliser.
Lorient climbed off the bottom of Ligue 1 on goal difference following a 2-1 home win against 10-man St Etienne.
Romain Philippoteaux opened the scoring for Lorient in the first half and substitute Jimmy Cabot struck in the final minute before Vincent Pajot headed a stoppage-time consolation for the visitors.
St Etienne goalkeeper Jessy Moulin was shown a straight red card in the 10th minute for bringing down Lorient striker Benjamin Moukandjo outside the penalty area.
Christopher Jullien’s late equaliser secured a share of the points for Toulouse as they claimed a 1-1 draw at home to Nancy.
The hosts moved up to seventh in the league after leaving it late to level against in-form Nancy, who enjoyed the bulk of the ball.
It was Nancy who took the lead just after the half-hour mark, with Maurice Dale holding the ball up in the box before laying it off for Youssef Ait Bennasser, who finished smartly into the corner.
Despite being on the back foot, Jullien equalised with five minutes to go with the towering header to claim a point.
Montpellier cruised to their biggest win of the season in Ligue 1 with a 4-0 home victory against 10-man Bordeaux.
The home side led 2-0 at half-time thanks to goals from Paul Lasne and Stephane Sessegnon, while Bordeaux striker Jeremy Menez was shown a straight red card following an off-the-ball incident late in the first period.
Steve Mounie and Yacouba Sylla added two more late goals for Montpellier, who also hit the crossbar through Lasne’s shot in the first half, to cap a miserable night for the visitors.
Elsewhere, Caen’s match against Metz at Stade Michel d’Ornano has been postponed until Sunday due to fog.
Caen’s match at Nantes last weekend was postponed for the same reason and they sit second-bottom in the standings after playing just one match so far in December – a 3-3 draw against Dijon.