Monaco suffered a surprise 1-0 defeat away to Evian on Saturday to leave Paris Saint-Germain 13 points clear at the top of the Ligue 1 table with just seven games left.
Just three days after trouncing Lens 6-0 in the quarter-finals of the French Cup, the principality club slipped to just their fourth league loss of the campaign as Cedric Mongongu's 86th-minute penalty made the difference.
The Congolese defender, who played for Monaco at the start of his career, sent Danijel Subasic the wrong way from the spot after Ricardo Carvalho blocked a Cedric Barbosa shot with his arm inside the area.
"This is a cruel result," said Monaco coach Claudio Ranieri.
"Evian defended very well and played on the counter-attack. But it's normal that a team battling to avoid relegation plays in this way."
Earlier, Monaco had created the best of the few chances there were in open play, but James Rodriguez was guilty of one particularly glaring miss as he failed to properly connect with Valere Germain's low cross with the goal gaping.
Evian, who drew in Monaco in November and won 2-0 at home to PSG in December, move five points clear of the relegation zone after this result while Ranieri's men may find their advantage over third-placed Lille reduced to six points if LOSC win against Guingamp on Sunday.
Cameroon international defender Nicolas Nkoulou scored a last-minute equaliser to earn Marseille a 1-1 draw at Sochaux, but it was a result that did little good to either side.
OM finished March with just two points from five games and they remain sixth, adrift of the European places, while Sochaux are five points from safety after failing to hold on to the first-half lead given to them by Stoppila Sunzu.
The Zambian international defender rose unmarked to head home Sebastien Roudet's right-wing corner in the 25th minute to set Sochaux seemingly on their way to a fifth successive win at their own Stade Bonal.
- Fans turn their backs -
Marseille offered little in terms of a response, and many of the travelling support made clear their displeasure with their team's slump by turning their backs on the action, but they were given something to shout about when Nkoulou - one of a number of OM players who are likely to be sold in the summer - headed home as the hosts struggled to clear following a Florian Thauvin corner.
Toulouse missed the opportunity to leapfrog Marseille into the top six as they were held to a 2-2 draw by Ajaccio in Corsica.
The visitors had fallen two goals behind as Ricardo Faty and Gadji Tallo netted for Ajaccio, but Issiaga Sylla pulled one back before Clement Chantome, the on-loan PSG midfielder, equalised.
But Toulouse missed a great chance to win the game when Jonathan Zebina saw a stoppage-time penalty saved by Guillermo Ochoa in the home goal.
Elsewhere, Odair Fortes scored to earn Reims a 1-1 draw at home to Lorient, for whom Vincent Aboubakar netted a first-half penalty, while there were goalless stalemates between Montpellier and Valenciennes and Nantes and Bordeaux.
On Friday, champions PSG took another step towards retaining their title and warmed up for next Wednesday's Champions League quarter-final first leg at home to Chelsea by winning 1-0 at Nice.
A Timothee Kolodziejczak own goal early in the second half at the Allianz Riviera allowed the capital club to stretch their club record-breaking run of consecutive wins in all competitions to nine.
The pick of the Sunday games sees Lyon meet Saint-Etienne in the Rhone derby, which will have a major bearing on the battle for European qualification.