Castres recorded a Top 14 double over Racing 92 as they claimed a hard-fought 13-9 victory in Paris on Saturday.
Having already beaten the former league leaders in November, Castres inflicted a first home defeat on Racing this season, a result that sees them slip to second.
Benjamin Urdapilleta was the Castres hero as he kicked eight points, while Julien Caminati scored the decisive try for the visitors on the half-hour mark.
The boot of Dan Carter kept Racing 92 in the match but they were unable to find a way through the resilient Castres defence.
Toulon are the new leaders after they earned a bonus-point win over Brive, their 44-15 victory moving them two points clear having played a game more than Racing.
The defending European champions ran in six tries, with Jonathan Pelissie kicking four conversions and two penalties in the comfortable triumph.
Clermont kept their Top 14 title hopes alive as they put a shaky start behind them to beat Oyonnax 44-16, Morgan Parra kicking 22 points to keep them within three of leaders Toulon.
Only five points separated the two sides at half-time, but tries from Wesley Fofana, Noa Nakaitaci, Isaia Toeava and Alexandre Lapandry saw them crush the strugglers in the second period.
Oyonnax slip to the foot of the table after the defeat due to Agen overturning a six-point deficit in the second half to beat La Rochelle 31-27.
Trailing 20-14 at the break, Agen scored two converted tries and a penalty as they recorded a first league win since October.
Last season's champions Stade Francais suffered their ninth defeat of the campaign as they were resoundingly beaten 33-18 by Grenoble, with hooker Arnaud Heguy scoring twice.
Meanwhile, Bordeaux strengthened their European chances with a 15-3 win over Section Paloise, their fourth successive victory.