An unbeaten 26-game run ended at Evian on Wednesday night as late goals from Clarck Nsikulu and Modou Sougou condemned PSG to a 2-0 defeat.
It was also the first time the capital club, who had plundered 34 goals in 15 previous games, failed to score in a league match this season.
Laurent Blanc's men still lead Lille by a point, though, with ambitious Monaco a further point behind in third, and next up at the Parc des Princes are struggling Sochaux.
Les Lionceaux are seven points adrift of safety having won only once, while their midweek 2-0 home defeat to Reims was a fifth reverse in eight games.
PSG striker Zlatan Ibrahimovic is, however, urging his team-mates to avoid thoughts of complacency and to instead make amends for the Evian debacle at the first time of asking.
"We played very poorly and were unable to lift ourselves," said the Swede.
"There's nothing more to say.
"We have to play better in our next game. We hadn't lost for a long time.
"Maybe this will be a good thing in the long term. We have to stay focused on our objectives."
Midfielder Adrien Rabiot added: "After a defeat, the most important thing is to bounce back straight away.
"We are still top of the league, so now we need to defeat Sochaux on Saturday."
Should PSG slip up, second-placed Lille could capitalise with a positive result at Bordeaux on Sunday.
Monaco could also go top with victory over relegation-threatened Ajaccio at Stade Louis II.
ASM coach Claudio Ranieri said: "There's a lot of pressure on PSG, it must be difficult for them.
"They're a team that could win the Champions League while it's our first year back at this level.
"We have to continue without worrying about what others are doing."
Marseille are 10 points off the pace in fourth but could both close the gap on the leaders and pull clear of fifth-placed Nantes when the Canaries come to town on Friday night.
Montpellier begin life without coach Jean Fernandez after the well-travelled tactician resigned this week.
The 2012 champions have won just twice this term and are five points clear of the drop zone following a 2-0 home defeat to Lorient, who host Brittany rivals Rennes.
Fernandez made the decision to step down on Thursday morning and Pascal Baills has been entrusted with the first team while Montpellier owner Louis Nicollin assesses candidates.
Lyon, in 13th, are another former title winner bidding to climb the table and they next travel to Corsica with a view to bringing upwardly-mobile Bastia back to earth.
Elsewhere, St Etienne target the top five when hosting PSG's conquerors Evian, Reims bid to establish themselves in the top 10 at the expense of faltering Nice, and surprise package Guingamp visit 18th-placed Valenciennes.