La Rochelle's hopes of wrapping up a French Top 14 semi-final spot suffered a setback Saturday when Lyon clinched a 29-25 victory, ending the league leaders' five-month undefeated run.
Argentine backrow forward Facundo Isa was the star of the show as he scored two of his team's five tries in the last seven minutes to help overturn a 17-22 deficit.
La Rochelle remain top on 80 points, 10 ahead of Clermont, with just two games of the regular season left to play.
They remain comfortably placed to reach the last four despite Saturday's loss, their first since November which also ended their bid to beat the tournament record of 11 games in a row unbeaten.
Richard Cockerill continued his perfect start as Toulon head coach as the three-time European champions boosted their play-off hopes with a 23-14 win over Castres.
Meanwhile, second-placed Clermont ran in nine tries in a crushing 59-18 bonus point victory away to Grenoble that boosted their hopes of getting a bye straight into the semi-finals while relegating their hosts to the second division alongside Bayonne.
Englishman Cockerill's first match in charge as interim Toulon boss until the end of the season ended in a 33-23 win over Toulouse last week.
"I'm very happy. It's a four points to zero win," said Cockerill, whose team moved into third sport, level on points with Montpellier.
"In this championship run-in all the teams are fighting for a home quarter-final."
A fourth defeat in a row was a tough blow for Castres, who face a midweek clash with Stade Francais in a late-season fixture pile-up caused by last month's player strike over the aborted fusion of Parisians Stade and Racing 92.
Victory for Toulon, the 2014 French champions, was built on a stirring first-half performance.
Wales full-back Leigh Halfpenny kicked over a penalty before quick hands in the backs saw the ball transferred from Ma'a Nonu to Mathieu Bastareaud and then Fiji wing Josua Tuisova, who twisted out of a pair of weak tackles before strolling over the tryline almost apologetically on 20 minutes.
Castres were pushing to get back into the game but a no-look pass in contact by Thomas Combezou was picked off by the alert Bryan Habana, and once the South African flyer set off on a 90-metre sprint to the line, no-one was going to catch him.
British and Irish Lions star Halfpenny converted both tries and added a second penalty to give Toulon a commanding 20-0 half-time lead.
- Different proposition -
Castres were a different proposition after the break and although wing Julien Caminati had a try ruled out for a forward pass, the visitors worked an overlap giving former Toulon wing David Smith space to beat Halfpenny and dot down.
South African-born France international scrum-half Rory Kockott converted the Samoan's score.
Halfpenny added another penalty but Castres were by now the better side, and after seeing another try refused by the video referee they scored a second try as Toulon creaked at their own scrum and Australian-born Tonga flanker Steve Mafi intercepted scrum-half Sebastien Tillous-Borde's desperate, fumbled pass before strolling over unopposed.
It was an 11-try fest in Grenoble, where eight Clermont players crossed the whitewash, including New Zealander Fritz Lee, Australian John Ulugia, Fijians Alivereti Raka and Setariki Tuicuvu, and Englishman David Strettle as Clermont warmed up for next weekend's European Champions Cup semi-final against Leinster in style.
Julien Arias and Geoffrey Doumayrou both scored a brace of tries as Stade Francais earned a third straight win, 51-16 over Pau, that moved them into a play-off spot, something that seemed unimaginable only a few weeks ago.
Fiji wing Waisea Nayacalevu, Italy captain Sergio Parisse and a penalty try boosted Stade as Pau's own top-six hopes suffered a massive blow.