Lorient, who just escaped relegation last season, stunned a visit to Monaco 1-0 on Friday when Nigerian striker Terem Moffi converted a penalty won.
Lorient made the most of opening the league on the second weekend of the season by jumping to the top of the rankings by four points.
Monaco, which finished third last season for a chance at a spot in the Champions League, left a number of older players on the bench on Tuesday for their Champions League qualifier against Shakhtar Donetsk.
Moffi was knocked down in the 31st minute by Monaco summer signing Ismail Jakobs and took the penalty to score. Moffi was Lorient’s top scorer last season with 14 goals, three of which were against Monaco.
Monaco dominated possession, but struggled to create clear chances. The few hits they managed to score were blocked by form goalkeeper Paul Nardi.
In front of more than 12,000 spectators, delighted to be back at the Moustoir stadium after nearly a season behind closed doors, and more and more excited as victory approached, Lorient maintained their small lead.