La Rochelle will have the chance to open up a gap at the Top 14 summit when they visit Toulouse on Sunday, after leaders Clermont Auvergne went down 26-16 to Castres.
Anthony Jelonch's 68th-minute try proved decisive at Stade Pierre-Antoine, with impressive replacement Benjamin Urdapilleta converting and then adding a late drop-goal.
Clermont's failure to claim even a losing bonus point means they remain level on points with La Rochelle at the top of the table.
Ben Barba made his rugby union debut in a losing cause as Toulon were beaten 15-5 at Brive.
However, the former Cronulla Sharks full-back could hardly be blamed for Toulon's defeat, given he only entered the fray with four minutes remaining, shortly after Bryan Habana had scored a consolation try.
Thomas Laranjeira landed four penalties for Brive, after Gaetan Germain's three-pointer had opened the scoring.
The two sides at the foot of the table each claimed notable results on Saturday.
Bayonne claimed a 25-25 draw against a Pau side that had won their previous five matches, both sides managing a converted try and six penalties apiece.
Grenoble went one better, capitalising on the fifth-minute sending-off of Chris Masoe to beat Racing 92 19-10. Masoe was dismissed for a dangerous tackle on Arnaud Heguy.