Monaco missed the chance to go back to the top of Ligue 1 on Saturday after playing most of the game with 10 men in a 3-1 defeat at Lorient, 24 hours after Marseille went to the summit by beating Strasbourg.
Captain Thilo Kehrer was sent off for a second yellow card late in the first half for Monaco in Brittany, and Lorient went ahead moments later courtesy of a Mohamed Bamba goal.
Substitute Pablo Pagis then scored a late brace to make sure of a fine win for newly-promoted Lorient, which lifts them into mid-table.
On-loan Barcelona forward Ansu Fati pulled one back for Monaco with a stoppage-time penalty, but his fourth goal in the last three games in all competitions was scant consolation.
Monaco could have reclaimed first place in the table at least provisionally by taking just a point from the game, having started the day as one of five teams locked on 12 points at the top of the table.
The principality side – for whom Paul Pogba is still yet to make his debut after arriving in the close season – have worryingly now let in nine goals in their last three matches as they prepare to host Manchester City in the Champions League in midweek.
Marseille came from behind to win 2-1 at Strasbourg on Friday with Amir Murillo scoring the decisive goal in stoppage time, meaning they top the standings on goal difference from Paris Saint-Germain, Monaco, Lyon and Strasbourg.
However PSG can go clear in the lead when they play Auxerre in the capital later, as they look to bounce back from suffering their first defeat of the campaign on Monday in a 1-0 loss at Marseille.