The Parisians beat Lorient 3-1 on Friday thanks to a hat-trick from their giant Swedish striker Zlatan Ibrahimovic.
The result leaves PSG on 59 points from 30 games with Lyon, who had taken the Ligue 1 lead from Marseille in the first game of 2015, second a point behind.
Marseille, who are five points behind PSG in third, travel to Lens on Sunday night.
If Marseille win they would draw to within a point of Lyon, and two of PSG, to send Ligue 1 into a three-way battle of nerves with just eight games to go.
Lyon coach Hubert Fournier accepted his side, rich in players from their youth academy, might have made a critical blunder.
“We’ve lost three points at a crucial part of the season,” he said. “And you can’t just say ‘Oh we’ll get them back later on’.”
“But how can you be angry when your forward line has an average age of 21 years, and your entire team lacks the depth of experience to deal with the pressure of that kind of game,” he added.
“We knew it could be tricky with Nice battling relegation, while we were playing without some of our senior players.”
– reduced to ten men –
Lyon had their star striker and talisman Alexandre Lacazette suspended for the match but visitors Nice were reduced to ten men when Lloyd Palun was sent off on 56 minutes.
The resulting penalty saw Maxime Gonalons equalise from the spot after the visitors had gone ahead when Carlos Eduardo converted a corner in acrobatic fashion on 23 minutes.
But a late penalty saw Nice reclaim the lead on 86 minutes with Valentin Eysseric stepping up to clinch three points and topple Lyon from the summit.
Claude Puel, the ex-Lyon coach now at the helm of Nice, had no sympathy for his former club as his Riviera outfit notched up vital points in the battle against relegation.
“The relegation battle is still going to be close,” said Puel, whose side are now provisionally five points above the first relegation spot.
“And in terms of the title race that’ll slow Lyon down and I’m happy about that because it means we won, and won with quality football despite everything.”
New Toulouse coach Dominique Arribage saw his relegation-threatened charges collect three potentially crucial points with a 2-1 home win against Bordeaux featuring a fine 18th-minute opening goal from Wissam Ben Yedder.
Bordeaux coach Willy Sagnol stuck with the side that shocked Paris Saint-Germain 3-2 last weekend, and they reduced the deficit ten minutes later through Uruguayan international striker Diego Rolan.
But Cameroonian international defender Jean-Armel Kana-Biyik headed in the winner for the hosts after 61 minutes after picking up a pass from Swiss left back Francois Moubandje that left Cedric Carrasso with no chance in the Bordeaux goal.
Toulouse are still third from bottom but close now to a clutch of teams above them.
Elsewhere Evian beat Montpellier 1-0 thanks to a goal six minutes from time from French-born Congolese striker Clarck N’Sikulu.
Midtable Nantes were held to a goalless draw at fellow Brittany side Rennes, one point below them, as Bastia and Caen also had stalemates at home at the hands of visiting Guingamp and Metz respectively. – Agence France-Presse