Biarritz took a step towards safety with victory in Lyon on Saturday while Perpignan slipped closer to the drop zone despite claiming a losing bonus-point in Clermont.
Toulon convincing beat Racing Metro to take a step closer to a home play-off.
Stade FranÃ§ais climbed back into the play-off positions (provisionally) with a bonus-point win over Agen.
Castres leapfrogged Montpellier into fourth spot in the standings with a crucial home win.
In Saturday's late game, Bayonne set up an almighty relegation showdown with Perpignan next week by grinding past 14-man Brive.
With Toulouse going down to a shock loss in Bordeaux on Friday, Clermont now trail the leaders by a single point.
Bordeaux-BÃ¨gles 18-17 Toulouse
Castres 27-18 Montpellier
Clermont 29-23 Perpignan
Lyon 17-34 Biarritz
Stade FranÃ§ais 53-27 Agen
Toulon 32-20 Racing MÃ©tro
Bayonne 19-12 Brive
Castres Olympique 27-18 Montpellier HR
Stade Pierre Antoine, Castres
Castres led 14-5 at half time.
Castres ended a three-game winless streak thanks to a great defensive effort to regain fourth place (synonymous with a home play-off) from their visitors.
Montpellier did most of the playing but failed to turn their territorial domination into points at crucial times.
Castres led 14-5 at half time thanks to a Marc Andreu try from turnover ball. Lucas Amorosino replied almost immediately for Montpellier but Romain Teulet's boot ensured the hosts held the advantage at the break.
Romain Martial's touchdown early in the second period gave CO a healthy lead despite struggling at scrum time.
Alex Tulou's try proved too little, too late for Montpellier, who head home empty handed.
Tries: Andreu, Martial
Pens: Teulet 5
Tries: Amorosino, Tulou
Cons: Bustos Moyano
Pens: Bustos Moyano 2
Castres: 15 Romain Teulet 14 Romain Martial, 13 James So'oialo, 12 Romain Cabannes, 11 Marc Andreu, 10 Pierre Bernard, 9 Thierry Lacrampe, 8 Chris Masoe, 7 Yannick Caballero, 6 Ibrahim Diarra, 5 Joe Tekori, 4 Matthias Rolland, 3 Luc Ducalcon, 2 Mathieu Bonello, 1 Yannick Forestier.
Replacements: 16 Marc-Antoine Rallier, 17 Michael Coetzee, 18 Jannie Bornman, 19 Steeve Malonga, 20 Rory Kockott, 21 Seremaia Bai, 22 Max Evans, 23 Karena Wihongi
Montpellier: 15 Lucas Amorosino, 14 Yoan Audrin, 13 Geoffrey Doumayrou, 12 Santiago Fernandez, 11 Martin Bustos Moyano, 10 FranÃ§ois Trinh-Duc, 9 Julien Tomas, 8 Mamuke Gorgodze, 7 Alex Tulou, 6 Fulgence Ouedraogo (captain), 5 Thibaut Privat, 4 Drikus Hancke, 3 George Jgenti, 2 Agustin Creevy, 1 Na'ama Leleimalefaga
Replacements:16 Erasmus Jansen van Vuuren, 17 Juan Figallo, 18 Alikisio Fakate, 19 Vassili Bost, 20 BenoÃ®t Paillaugue, 21 Paul Bosch , 22 Timoci Nagusa, 23 Maximiliano Bustos
ASM Clermont Auvergne 29-23 USA Perpignan
Parc des Sports Marcel Michelin, Clermont Ferrand
USAP gave their hosts a real run for their money but ultimately paid for their indiscipline.
Clermont were made to work very hard to retain their unbeaten record at home (their 40th successive win at their fortress) against a combative Catalan side that are fighting for their Top 14 lives.
The result leaves Clermont one point short of the championship lead and USAP teetering on the edge of the relegation zone.
Perpignan were far cry from the side that has struggled all year and were rewarded with two tries for their aggression at the breakdown zone.
But in the end David Skrela's boot won the day for the hosts as he landed eight penalties.
Clermont took their opportunity despite a bruising start that saw their prop Lionel Faure and Perpignan hooker Charles Geli both yellow-carded for a second-minute punch-up.
Alexandre Lapandry scored early for Clermont but the hosts replied with a try from full-back Joffrey Michel to leave the scores at 17-13 to the home side.
Adrien Plante touched down to level matter early in the second half but Skrela kept the scoreboard ticking.
A late penalty from James Hook secured a vital losing bonus point for USAP, who face a relegation showdown with Bayonne next week.
"I am satisfied, especially by our tactics," said Clermont coach Vern Cotter.
"It was important to have that sort of commitment ahead of our European Cup quarter-final tie."
Pens: Skrela 8
Tries: Michel, Plante
Cons: Hook 2
Pens: Hook 3
Yellow cards: Faure (Clermont - 1st min); Geli (Perpignan - 1st min); Perez (Perpignan - 46th min)
Clermont: 15 Lee Byrne, 14 Sitiveni Sivivatu, 13 Noa Nakaitaci, 12 Regan King, 11 Jean-Marcellin Buttin, 10 David Skrela, 9 Morgan Parra, 8 Elvis Vermeulen (captain), 7 Alexandre Lapandry, 6 Julien Bardy, 5 Julien Pierre, 4 Jamie Cudmore, 3 Danie Kotze, 2 Tii Paulo, 1 Lionel Faure.
Replacements: 16 BenoÃ®t Cabello, 17 Vincent Debaty, 18 Loic Jacquet, 19 Jason White, 20 Ludovic Radosavljevic, 21 Brock James, 22 Brent Russell, 23 Davit Zirakashvili.
Perpignan: 15 Joffrey Michel, 14 Adrien PlantÃ©, 13 David Marty, 12 Maxime Mermoz, 11 Julien Candelon, 10 James Hook, 9 Florian Cazenave, 8 Ovidiu Tonita, 7 Jean-Pierre Perez, 6 Bertrand Guiry, 5 Romain Taofifenua, 4 Sebastien Vahaamahina, 3 BenoÃ®t Bourrust, 2 Charles Geli, 1 Kisi Pulu.
Replacements: 16 Guilhem Guirado, 17 Perry Freshwater, 18 Gerrie Britz, 19 GrÃ©gory Le Corvec, 20 David MÃ©lÃ©, 21 Nicolas Laharrague, 22 Jean-Philippe Grandclaude, 23 Romain Brison.
Lyon Olympique 17-34 Biarritz Olympique
Stade Matmut, Lyon
Biarritz will breathe a massive sigh of relief as the climb up to tenth in the standings with their third consecutive win.
Lyon on the other hand looked condemned to relegation.
The scores were locked at 10-all at half time and a repeat of the 15-all draw earlier in the season looked on the cards after LOU scrum-half Ricky Januarie score his first Top 14 try.
But in the second half the power of Biarritz's returning internationals came to the fore as skipper Imanol Harinordoquy and Dimitri Yachvili both scored.
Tries: Januarie, Salobert
Cons: Loursac, Thomas
Tries: Ngwenya, Harinordoquy, Haylett-Petty, Yachvili
Cons: Yachvili 4
Pens: Yachvili 2
Yellow cards: Januarie (Lyon - 53rd min)
Lyon: 15 Romain Loursac, 14 Franck Romanet, 13 Vinaya Wakanivuga, 12 Nicolas Raffault, 11 RÃ©my Grosso, 10 Laurent Tranier, 9 Ricky Januarie, 8Sisaro Koyamaibole , 7 Juan LeguizamÃ³n, 6 Eugene N Zi, 5 Coenie Basson, 4 Arnaud Marchois,