Arnaud Kalimuendo scored against his boyhood club as leader Paris Saint-Germain was beaten 2-0 by Rennes in its first home loss in the French league this season.
The win ended PSG's 35-match unbeaten run in the league at the Parc des Princes.
Marseille later won 2-1 to end Reims' 19-match unbeaten run and cut PSG's lead to seven points with 10 matches left.
The 21-year-old Kalimuendo joined PSG's youth system when he was 9 years old and signed with Rennes last summer looking for more playing time away from the armada of PSG stars. Kalimuendo was under contract with PSG until 2024 but spent the last two seasons on loan at Lens.
Rennes, which had already prevailed in the corresponding fixture in Brittany two months ago, made the most of PSG's passivity and relied on goalkeeper Steve Mandanda's decisive saves to produce a clean sheet. Karl Toko Ekambi scored Rennes' first goal.
PSG looked slow and lethargic, relying in vain on the skills of Kylian Mbappe and Lionel Messi.
Mbappe had two good chances to break the deadlock in the first half but the France star was denied twice.
Mandanda stopped Mbappe's chipped effort in the 26th minute. The former France 'keeper then blocked Mbappe's shot from Messi's deep pass with a superb reflex save in the 40th.
Rennes waited patiently and took its chance at the stroke of halftime when Benjamin Bourigeaud picked out Toko Ekambi behind the defense. The Cameroon forward controlled the ball with a delicate touch and beat Gianluigi Donnarumma with an angled shot.
PSG players were whistled off the field by some fans, and things got worse when they returned from the locker room.
Kalimuendo made it 2-0 in the 48th after PSG clumsily gave the ball away in midfield. He did not celebrate his goal.
PSG pressed harder as it tried to pull one back but made too many technical mistakes and was vulnerable to counterattacks, with Rennes having the best chances after the interval.
Mandanda was decisive again 10 minutes from time to firmly parry away Marco Verratti's shot that looked bound for the goal.
PSG is out of the Champions League and the French Cup, meaning it has just the league title left to focus on. Chasing a European spot, Rennes moved up to fifth place in the standings.
MARSEILLE IN THE HUNT
Second-place Marseille's hard-fought victory in the Champagne town marked its eighth straight away win.
Marseille forward Alexis Sanchez scored twice against Reims, including a superb curled free kick, after Folarin Balogun put the hosts in the driving seat.
Reims pushed hard for an equalizer in the second half and Marseille goalkeeper Pau Lopez made several decisive saves to keep his team ahead. Marseille was lucky not to concede again as the hosts hit the woodwork twice.
BEN YEDDER HITS 80 MARK
Wissam Ben Yedder scored his 80th career league goal for Monaco in a 2-0 win at 10-man Ajaccio as the Principality side got back to winning ways in the top flight.
It was Ben Yedder's 17th league goal this season. The France striker put the hosts ahead in the 26th minute before Krepin Diatta sealed Monaco's win with seven minutes left.
Ajaccio was forced to play with 10 players for most of the second half after Cyrille Bayala was sent off for a reckless tackle on Caio Henrique in the 51st minute.
Fourth-place Monaco had picked up only one point from its previous three matches.
Ben Yedder became the third player in Monaco's history to score 80 league goals for the club after Delio Onnis (157) and Lucien Cossou (97).
He scored from a rebound from close range following a free kick from Henrique. Monaco was then in control against a poor Ajaccio side that was punished by Diatta on the counter and remained in the relegation zone.
WAHI'S DOUBLE
Montpellier's revival under coach Michel Der Zakarian continued as the Mediterranean club came from behind to beat Clermont 2-1 and moved to 11th place.
Montpellier is now unbeaten in six matches, including five wins, since Der Zakarian returned in February.
Montpellier had a chance to take the lead in the 13th minute but Stephy Mavididi hit the post. Saïf-Eddine Khaoui then gave Clermont the lead before the up-and-coming Wahi turned things round, first with a header then a precise shot. Wahi has 12 league goals, his best tally over a single season.
NICE STAYS UNBEATEN
Seventh-place Nice extended its 11-game unbeaten run but was held to a 1-1 draw by Lorient.
Bamo Meite scored the first goal of his professional career to give the visitors the lead before Gaetan Laborde put the teams level.
OTHER RESULTS
Strasbourg beat Auxerre 2-0 to end a three-game winless run and Brest drew 2-2 at Troyes.
