Paris Saint-Germain lost at AS Monaco 1-0 on Saturday for their second defeat of the season after 14 rounds, as many as all last season for the defending Ligue 1 champion.
Japan forward Takumi Minamino scored midway through the second half for Monaco against an unusually sloppy PSG side.
"It's hard to maintain consistency when you make individual errors. It was our worst match of the season," PSG coach Luis Enrique said.
"They deserved it more than we did. It was a bad night for us, a very bad night."
Portugal midfielder Vitinha, who scored a brilliant hat trick in the Champions League in midweek, was kept quiet by Monaco's hard-working midfield.
"It was difficult tonight. When you play a match full of imprecision and mistakes, it's impossible to win. We have to accept the level we showed today," Luis Enrique added.
PSG was exposed to counterattacks and Minamino made no mistake in the 69th, expertly controlling Aleksandr Golovin's pass from the left and clipping the ball past Chevalier.
Monaco midfielder Lamine Camara was fortunate to escape with a yellow card early on for a dangerous lunge on goalkeeper Lucas Chevalier, violently catching Chevalier's right ankle just after he kicked the ball.
Luis Enrique reacted to the controversial incident involving Chevalier, saying, "Lucas Chevalier was very lucky today. I won't say more because it's not my job, but it's incredible. He was very lucky." Chevalier recovered and made a good one-handed save from Minamino's volley later in the first half.
Monaco defender Thilo Kehrer was sent off in the 80th minute following a video review after shoving a PSG player in the back. Former midfield star Paul Pogba came off the bench for Monaco in the closing stages.
PSG kept first place in the standings after Marseille missed the chance to overtake them at the top of Ligue 1 later on Saturday.
An injury-time goal from Santiago Hidalgo gave Toulouse a 2-2 draw at the Velodrome.
However, Lens can take first place on Sunday with a win at Angers.
Information from The Associated Press was used in this report