A home loss to relegation-threatened Valenciennes in December was followed by a 1-1 draw at Montpellier last week.
And champions PSG capitalised on the slip-up made by Claudio Ranieri's side, as they surged five points clear at the top of the table with victory at Ajaccio last time out.
Toulouse are in good Ligue 1 form, unbeaten in six, and won 1-0 at Bordeaux on Saturday - a result that lifted them to 10th in the table.
Monaco striker Radamel Falcao has battled injury issues this season and has scored just once in his last four matches for the club, with Ranieri desperate for the Colombian forward to find form.
PSG host sixth-placed Nantes on Sunday and will be buoyed by their 3-1 Coupe de la Ligue win at Bordeaux on Tuesday.
Goals in the last five minutes from Adrien Rabiot and Blaise Matuidi helped the capital club into the last four, as their bid for a domestic treble gathered pace.
PSG won 2-1 at Nantes earlier this season thanks to Ezequiel Lavezzi's late strike and will enjoy returning to Parc des Princes, where they have not lost in the league since November 2012.
Third-placed Lille are now just two points behind Monaco and take their outstanding record to Saint-Etienne on Friday.
Lille have conceded just 10 goals in 20 Ligue 1 matches and will hope to make the most of Saint-Etienne's humiliating Coupe de France exit at fourth-tier Cannes on Tuesday.
Saint-Etienne, who sit fourth, had the chance to win the penalty shootout that followed a 1-1 draw, but lost their nerve as Cannes celebrated a famous triumph.
Marseille host troubled Valenciennes on Saturday, with the latter facing the prospect of being administratively relegated at the end of the season if their financial problems continue.
They might drop to Ligue 2 on form anyway, but victories in their last two league matches have boosted spirits and seen them rise to 18th.
Other fixtures on Saturday see Bastia take on Bordeaux, Lorient battle Guingamp and Nice face Ajaccio.
There are also matches between Rennes and Evian TG, and Sochaux and Montpellier, while a slowly-improving Lyon hit the road to face Reims on Sunday.