Marseille passed up the chance to go level on points with Ligue 1 leaders Paris Saint-Germain on Sunday after losing at Strasbourg, while Lyon snatched a 2-1 victory at Toulouse thanks to a 95th-minute winner.
Under new Italian coach Roberto De Zerbi, Marseille had looked resurgent this season but Diego Moreira's strike for Strasbourg just before the break condemned the visitors to a first defeat of the season.
Strasbourg's first win in 17 outings against Marseille stopped their opponents from joining pacesetters PSG and Monaco on 16 points atop Ligue 1.
"I have complete confidence in my players," said De Zerbi.
"One of the club's former coaches, Marcelo Bielsa, used to say: 'You learn from defeat', and I think we're going to learn a lot tonight, it's going to be a decisive day for our progress."
In the 40th minute, Moreira surged onto a through ball on the left and slotted a calm finish past Marseille goalkeeper Geronimo Rulli to score the first goal of his senior professional career.
Marseille had chances to equalise but Djordje Petrovic in the Strasbourg goal was equal to Mason Greenwood as the home side saw out their second win of the campaign.
Earlier, Lyon also got their second win of the season courtesy of a Rasmus Nicolaisen own goal and Malick Fofana's stoppage-time winner, after Yann Gboho had sent Toulouse into an early lead.
The hosts went in front on 14 minutes when Gboho fired into the top corner after Lyon gave the ball away in their own half.
But Pierre Sage's side were back level two minutes before the half-hour when Nicolaisen turned into his own net.
It seemed the points would be shared until the ball broke to Fofana in the box and his flashed effort beat the goalkeeper and just crossed the line before defender Gabriel Suazo could block.
OTHER RESULTS
Reims beat Angers 3-1 on the road to move up to fourth, two points behind resurgent Marseille in third.
Goals by Keito Nakamura and Junya Ito inside 25 minutes gave Reims the ideal start and Marshall Munetsi then netted in the 79th minute to seal the three points.
Zinedine Ferhat scored a consolation for the hosts with six minutes remaining, but Angers remain rooted to bottom spot. They are the only team yet to win a match in the league this season.
A second-half brace from Ibrahim Sissoko earned a valuable away point for promoted Saint-Etienne against Nantes.
Johann Lepenant sent the home team in front on 10 minutes and Sorba Thomas doubled their lead four minutes after the break.
Sissoko halved the deficit in the 57th minute and then converted a penalty 10 minutes later to level.