The absence of talismanic World Cup winner Lionel Messi and the suspended Neymar proved to be an insurmountable obstacle for Paris Saint-Germain, who saw their unbeaten run since March 20 last year come to an abrupt end following a 3-1 loss to Lens on Sunday.
"Lens deserved their win," said PSG coach Christophe Galtier.
Lois Openda scored one and set up another as Lens, who have only lost once in Ligue 1 this season, closed the gap on PSG at the top to four points after 17 rounds. "We are happy to be four points behind. This victory proves that we can beat anyone in this championship," said Lens coach Franck Haise.
PSG's last loss was more than nine months ago when they fell 3-0 at Monaco in Ligue 1, although that still left them 12 points clear at the top of the table.
On Sunday at Lens, PSG were without Messi and suspended Brazilian Neymar, but Kylian Mbappe, the Golden Boot winner in Qatar, received a rousing welcome from Lens fans as he warmed up.
Galtier said he hoped French fans would give Messi a similar reception. "He's going to join us on Jan. 3, in 48 hours. I hope Leo will be well received. He won the most beautiful trophy by being very good. We know he's a great and decisive player so we hope he'll be well received obviously," said Galtier.
"Kylian worked hard. He exhibited great attitude. Leo and 'Ney' were not there but that is no excuse," added Galtier.
Lens took the lead after five minutes when PSG goalkeeper Gianluigi Donnarumma blocked an effort from Massadio Haidara to Przemyslaw Frankowski who scored.
Hugo Ekitike replied three minutes later, swinging hard as Brice Samba dived to gather a cross and driving the ball through the Lens goalkeeper's grasp.
Lens took control after 28 minutes. Seko Fofana won possession on the edge of his own box, drove through a couple of challenges, then measured a pass between the PSG central defenders to Openda. The Belgian sprinted clear, and squeezed a shot under Donnarumma.
Lens pressed from the start of the second half and in the 48th minute won possession deep in the PSG half. The ball reached Openda in the box, his backheel left the defense flat-footed and allowed Alexis Claude Maurice to take a touch and score from close range.
Celebration Week
The French league normally takes the holiday season off, but with matches to make up following the World Cup, it played one round immediately after Christmas and another on Sunday and Monday. They have dubbed the games "Celebration Week", in English, as a nod to the English tradition of Boxing Day games, but many fans are suspicious or hostile.
Among protest banners unfurled by fans in an almost empty stand at Nantes, where the home team beat Auxerre 1-0, was one that complained that the league and broadcasters were "killing" them.
In Monaco, Aleksandr Golovin smashed in the only goal from outside the penalty area as the home team did just enough to beat Brest 1-0 and tighten their grip on fourth place.
Angers remain anchored to the bottom after suffering a ninth straight loss, falling 2-1 to visiting Lorient who broke their own run of six games without a victory.
The home team's Moroccan World Cup stars, Sofiane Boufal and Azzedine Ounahi, who have just returned from vacation and reportedly been promised lucrative transfers, were honored before the match but did not appear. "I was happy to see Sofiane and Azzedine. For the moment they are here. For the rest, I leave that to the club directors. I only take care of what happens on the pitch," said Angers coach Abdel Bouhazama.