Paris Saint-Germain manager Laurent Blanc said he was shocked by his side's first loss in 47 matches in all domestic competitions on Sunday, and added he had no explanation for the 2-1 setback against Lyon.
PSG had not lost for 36 games in Ligue 1 -- a run stretching back almost 12 months -- but the French champions' hopes of an invincible season were destroyed by first-half goals from Maxwel Cornet and Sergi Darder.
Lucas Moura pulled one back for the runaway league leaders after 51 minutes, but Lyon were worthy winners as they climbed above Caen into third place on goal difference.
Though Blanc's side still hold a 23-point lead over second-placed Monaco and will undoubtedly retain their title, the French boss expressed his disbelief with the result afterward.
But Blanc told Canal+: "If I had an explanation, I would give it to the players. It was totally unexpected. We expected a difficult game, but not as hard.
"I took out a player at half-time [Benjamin Stambouli], but it could have been more. We must use this game to regroup."
PSG skipper Thiago Silva blamed defeat on a poor first-half performance.
"The second half was a little better but not much more," said Silva.
"All decisions we have taken on the ground were a bit amateurish and this defeat is inexplicable.
"We played five times against Lyon, won four and lost once, but now we must look ahead and forget this match."
Meanwhile, Lyon head coach Bruno Genesio believes his side will be inspired by ending PSG's long unbeaten record.
"This match counts for three points but it is a big satisfaction to beat PSG," Genesio said at his postmatch press conference. "This gives us a lot of hope for the rest of the championship.
"I am not surprised by the quality and the spirit of my team.
"What did I like? The tactical plan that was implemented, in particular the will to press them high, the technical quality, the attitude of the players, the selflessness with a young team.
"Now we have to stay down to earth, in the same way we did not panic when we were not playing so well."
Genesio succeeded Hubert Fournier on Christmas Eve and Lyon have now won four of their last five games.
They are also unbeaten at home since they left the Stade de Gerland and moved into their new Parc OL arena last month.
"Since the beginning of the year, I think we still have a consistency in the quality of our games -- even though it was not always rewarded," said Genesio.
"It feels very good here, we have tremendous support in difficult times. This evening for the last 15 minutes they gave us extra energy to keep this result."