Kylian Mbappe made his return to Paris Saint-Germain but went down 1-0 away to Rennes in Ligue 1 on Sunday.
This leaves them just three points clear at the top of the table halfway through the season.
Rennes captain Hamari Traore scored the only goal in the 65th minute.
Prior to that they had gone unbeaten through their first 23 matches this season in all competitions and this latest setback gives their rivals hope in the title race.
"I am disappointed with our performance because we conceded too many chances and created nothing," said PSG coach Christophe Galtier, who complained of a "total absence of a presence in the penalty box" after opting to leave Mbappe on the bench at kick-off.
