Marseille paid the price for the second-half dismissal of Joey Barton as they slipped to a shock 1-0 defeat at home to lowly Nancy.
The on-loan midfielder was shown two yellow cards in the space of five minutes to leave OM facing an uphill struggle at the Stade Velodrome.
And an own goal from Andre Ayew capped off a miserable evening for the title hopefuls, who remain six points behind Ligue 1 leaders Paris St Germain.
Adrian Mutu scored twice as Ajaccio inflicted a blow to Lyon's title hopes with a 3-1 victory.
Paris St Germain's three-point lead, gained by winning 4-0 in Toulouse on Friday, was sure to last the weekend after this surprising result.
Ajaccio were without a win in five league matches and perched precariously, just two points above the relegation zone, as Lyon arrived looking to rejoin PSG at the summit.
And when Lyon edged ahead early in the second half it was all going to plan for Remi Garde's men.
But Ajaccio were soon level, thanks to Chahir Belghazouani, and Romanian Mutu headed them ahead before adding a late penalty after Lyon missed a chance to equalise from the spot.
Reims remain in the Ligue 1 relegation zone after squandering a first-half lead in a 3-1 defeat at Montpellier.
Gaetan Courtet put the visitors in front on 33 minutes, but Montpellier equalised just three minutes into the second half through Souleymane Camara and went on to claim all three points thanks to goals from Marco Estrada and Younes Belhanda.
With none of the leading trio in action on Saturday, all eyes were on the battle for Europe behind them as Bordeaux, Nice and St Etienne all won to remain within a point of each other in the fight for fourth.
Bordeaux ensured they would hang onto fourth on goal difference as they saw off Valenciennes 2-0 with close-range finishes in each half from David Bellion and Ludovic Obraniak.
Nice, who lost to Bordeaux last weekend, are right behind, level on 38 points, having won 2-0 away to Brest.
Nemanja Pejcinovic put them ahead after 13 minutes, slamming home after Brest failed to deal with Xavier Pentecote's corner, and they doubled their advantage only seconds into the second half when Brahim Ferradj put through his own net.
St Etienne, one point behind in sixth, were grateful to the crossbar for preserving their 2-1 away win over relegation-battling Sochaux.
Sloan Privat's injury-time header came back off the bar as St Etienne hung on for a victory that would have been assured had Pierre-Emerick Aubameyang not missed an 81st-minute penalty.
Aubameyang had put the visitors in front with a powerful volley into the top corner after 22 minutes, but they missed chances to add to their lead before Edouard Butin levelled in the 71st minute.
Parity lasted only five minutes before Fabien Lemoine's powerful shot picked up a deflection on its way to goal, but Aubameyang's penalty miss left St Etienne hanging on.
That saw St Etienne move above Rennes on goal difference as les Rouges et Noirs needed a last-minute goal from substitute Axel Ngando, only on the pitch for a matter of seconds, to take a 2-2 draw away to Lorient, who are just two points further back in the congested upper mid-table.
Jeremie Aliadiere had put Lorient in front 10 minutes from time after Mevlut Erdinc cancelled out Yann Jouffre's opener, but just when Lorient looked poised to climb above Rennes in the table, Ngando scored with his first touch, slipping the ball through the goalkeeper's legs.
Lille slipped further off the pace as they were held to a 1-1 home draw by relegation-battling Troyes, needing Divock Origi's 74th-minute strike to cancel out Fabien Camus' first-half opener.
The result leaves them seven points off fourth down in 10th place.
Bastia and Evian played out a goalless draw.
On Friday, Paris St Germain responded to the much-publicised arrival of David Beckham with a comfortable 4-0 win at Toulouse to strengthen their position at the top.
The former England captain signed for the Parisian club until the end of the season but ruled himself out of this fixture because of a lack of match fitness, having not played since December.
However, his new team-mates proved they could do it without him and were already leading through goals from Javier Pastore and Zlatan Ibrahimovic when Toulouse were reduced to 10 men with the dismissal of Cheikh M'Bengue.
It was one-way traffic from that point onwards, and further goals from Mamadou Sakho and Gregory van der Wiel secured a seventh win in eight matches to lift PSG three points clear at the top.