A first-half goal by defender Jacques Faty and one near the end from forward Sloan Privat gave Sochaux victory but Lyon, who visit Liverpool in the Champions League on Tuesday, stayed a point ahead of Girondins Bordeaux, who lost 1-0 at Auxerre.
"This is frustrating but now we have to think about Liverpool," Lyon's Brazilian midfielder Ederson told reporters. "Of course, we would have preferred a win before going there but we need to bounce back straight away."
Bordeaux, who ended Lyon's seven-year supremacy by taking the title last May, missed a penalty through Argentine striker Fernando Cavenaghi in the first half and saw Slovenia forward Valter Birsa convert a second-half penalty for Auxerre.
"Things have not gone our way lately but what we have to do is get straight back to work," Bordeaux defender Mathieu Chalme told reporters. "The best way to get our confidence back would be to win in Europe."
Lyon were joined on 20 points from nine games by promoted Montpellier, who extended an unexpectedly strong run by beating visitors St Etienne 2-1. Bordeaux dropped to third place.
Like Lyon, champions Bordeaux, who host Bayern Munich on Wednesday, warmed up for Champions League duties in poor fashion, suffering their second consecutive Ligue 1 defeat.
The only side to get a boost ahead of their duties in Europe were Olympique Marseille, who bounced back from two league defeats in succession by winning 3-0 at Nancy.
"People were starting to have a go at us but we just held tight and showed tonight that we are a real team," Marseille midfielder Benoit Cheyrou told reporters.
The former European champions, who face FC Zurich in Switzerland in the Champions League on Wednesday, moved up to fourth place, three points off the pace.
