Top 14 leaders Toulon and second-placed Clermont slumped to shock defeats on Saturday as the runaway title rivals came unstuck in the Basque country.
Toulon, having rested a number of top stars, suffered a 33-28 defeat away to Bayonne who avenged a 59-0 rout earlier this season.
Clermont then wasted the opportunity to take pole position by squandering a 28-22 lead at Biarritz going into the last 10 minutes before losing 32-28.
Elsewhere, Montpellier became only the second team to win in Grenoble this season to keep their hopes of making the Top 14 play-offs alive.
Bordeaux-Bègles took a huge step towards avoiding relegation with a bonus-point win over Mont-de-Marsan, who can no longer avoid the drop.
Castres held onto fourth place with victory over Agen, who now look likely to lose their place in the top division, while Racing bagged their ninth straight win at home to Perpignan.
Sunday, March 24
Toulouse v Stade Français
Union Bordeaux-Bègles 40-7 Stade Montois
Stade Jacques Chaban-Delmas, Bordeaux
It's official: Mont-de-Marsan are relegated, with four rounds left to play.
UBB on the other hand now look set to stay up as they have a nine-point gap on Agen.
Tries: Reihana, Talebula, Le bourhis, Avei, Mailei, Sanchez
Cons: Lopez 5
Yellow cards: Fiorini, Arrayet
Bordeaux-Bègles: 15 Bruce Reihana, 14 Metuisela Talebula, 13 Felix Le bourhis, 12 Julien Rey, 11 Blair Connor, 10 Camille Lopez, 9 Heini Adams, 8 Matthew Clarkin, 7 Justin Purll, 6 Hugh Chalmers, 5 Adam Jaulhac, 4 Cameron Treloar, 3 Silviu Florea, 2 Wayne Avei, 1 Laurent Delboulbes,
Replacements: 16 Hikairo Forbes, 17 Jefferson Poirot, 18 Francois Tisseau, 19 Louis benoit Madaule, 20 Julien Seron, 21 Frederico nicolas Sanchez, 22 Andrew Mailei, 23 Tamato Leupolu.
Mont-de-Marsan: 15 Yohann Durquet, 14 Simon Lucu, 13 Jean baptiste Dubie, 12 Baptiste Chedal, 11 Julien Cabannes, 10 Bernard Arrayet, 9 Arnaud Pic, 8 Alexandre Ricaud, 7 Johannes Britz, 6 Julien Tastet, 5 Berend Botha, 4 Sebastian De chaves, 3 Sebastien Ormaechea, 2 Pieter Grobler, 1 Alexandre Menini,
Replacements: 16 Julien Janaudy, 17 Julian Fiorini, 18 Scott Murray, 19 Yann Brethous, 20 Antoine Vignau tuquet, 21 Daniele Baleinadogo, 22 Jean marc Mazzonetto, 23 Mathieu Giudicelli.
Castres Olympique 20-9 SU Agen
Stade Pierre Antoine, Castres
Castres regain the fourth-place they lost to Racing earlier in the day to keep the heat on Toulouse and stay in pole position for a home play-off.
CO led 13-9 at the break and will be disappointed to have not sealed the bonus point in the second half.
Agen, fighting for the Top 14 lives, were in the game until Yannick Forestier's 67th minute try.
Agen, who have a tough end-of-season schedule, are now clear favourites to join Mont-de-Marsan in the ProD2 next season.
Tries: Cabannes, Forestier
Cons: Kockott, Teulet
Pens: Kockott 2
Pens: Barnard 3
Castres:15 Romain Teulet, 14 Romain Martial, 13 Romain Cabannes, 12 Seremaia Baik, 11 Marc Andreu, 10 Remi Tales, 9 Rory Kockott, 8 Antonie Claassen, 7 Yannick Caballero, 6 Jan de bruin Bornman, 5 Rodrigo Capo ortega, 4 Christophe Samson, 3 Karena Wihongi, 2 Brice Mach, 1 Saimone Taumoepeau,
Replacements: 16 Mathieu Bonello, 17 Yannick Forestier, 18 Matthias Rolland, 19 Piula Faasalele, 20 Thierry Lacrampe, 21 Remi Lamerat, 22 Marcel Garvey, 23 Mihaita Lazar
Agen: 15 Mathieu Lamoulie, 14 Silvere Tian, 13 Benjamin Petre, 12 Miguel Avramovic, 11 Jean francois Coux, 10 Conrad Barnard, 9 Mathieu Loree, 8 Ueleni Fono, 7 Remi Vaquin, 6 Jean Monribot, 5 William Demotte, 4 Lisiate Faaoso, 3 Viliamu Afatia, 2 Semisi Telefoni, 1 Bernard N'nomo,
Replacements: 16 Jalil Narjissi, 17 John Schwalger, 18 Leo Bastien, 19 Pierre Julien, 20 Alexi Bales, 21 Vincent Roux, 22 Ben Seymour, 23 Gert Muller.
FC Grenoble 9-16 Montpellier Hérault Rugby
Stade Lesdiguières, Grenoble
A crucial result in the arce for the semi-finals as Montpellier climb above Perpignan into that all-important sixth place.
Grenoble saw their precious unbeaten home record broken by Clermont last month and nothing has gone right for them since. They can waves their chances of European qualification goodbye.
Second-choice France fly-half François Trinh-Duc proved a point by scoring the only try of the game in the 16thn minute to help the visitors to a 7-6 lead at the break.
Grenoble get a losing bonus point but will harbour regrets after Valentin Courrent landed just two of his five attempts at goal.
Pens: Courrent 2, Stewart
Yellow cards: Sowerby, Mutapcic
Pens: Paillaugue 2, Bustos Moyano
Yellow card: Fakate
Grenoble: 15 Fabien Gengenbacher, 14 Viliame Waqaseduadua, 13 Rudolf Coetzee, 12 Nigel Hunt, 11 Florian Ninard, 10 Blair Stewart, 9 Valentin Courrent, 8 Shaun Sowerby, 7 Roland Bernard, 6 Jonathan Best, 5 Andrew Farley, 4 Benjamin Hand, 3 Dayna Edwards, 2 Anthony Hegarty, 1 Kenan Mutapcic,
Replacements: 16 Romain David, 17 Vincent Campo castillo, 18 Fabien Alexandre, 19 Florent Fourcade, 20 Rida Jaouher, 21 Aaron Bancroft, 22 Jonathan Pelissie, 23 Albertus Buckle
Montpellier: 15 Pierre Berard, 14 Yohann Artru, 13 Thomas Combezou, 12 Paul Bosch, 11 Yoan Audrin, 10 Francois Trinh-Duc, 9 Benoit Paillaugue, 8 Alex Tulou, 7 Alexandre Bias, 6 Fulgence Ouedraogo, 5 Thibaut Privat, 4 Alikisio Fakate, 3 Maximiliano Bustos, 2 Agustin Creevy, 1 Mikheil Nariashvili,
Replacements: 16 Charles Geli, 17 Yvan Watremez, 18 Hendrikus Hancke, 19 Kelian Galletier, 20 Julien Tomas, 21 Hamish Gard, 22 Martin Bustos moyano, 23 Barry Faamausili,
Biarritz Olympique 32-28 ASM Clermont Auvergne
Parc des Sports d'Aguilera, Biarritz
Clermont had been 19-13 down at the interval against Biarritz at the Stade d'Aguilera, but grabbed the lead through two tries in three second-half minutes from Brock James and Davit Zirakashvili for a 28-22 advantage.
However seven minutes from time, Biarritz centre Benoit Baby scored his team's second try with Dmitri Yachvili converting for a 29-28 lead which became 32-28 with a last-minute penalty.
Scrum-half Yachvili finished with 22 points while Clermont were left with the slender consolation of a losing bonus point.
Biarritz: 15 Jean pascal Barraque, 14 Takudzwa Ngwenya, 13 Benoit Baby, 12 Damien Traille, 11 Aled Brew, 10 Julien Peyrelongue, 9 Dimitri Yachvili, 8 Imanol Harinordoquy, 7 Raphael Lakafia, 6 Thibault Dubarry, 5 Mathias Marie, 4 Erik Lund, 3 Francisco Gomez Kodela, 2 Arnaud Heguy, 1 Eugene Van staden,
Replacements: 16 Romain Ruffenach, 17 Thomas Synaeghel, 18 Pelu ian Taele, 19 Benoit Guyot, 20 Yann Lesgourgues, 21 Paul Couet lannes, 22 Marcelo Bosch, 23 Benedict Broster
Clermont: 15 Jean marcellin Buttin, 14 Sitiveni Sivivatu, 13 Aurelien Rougerie, 12 Regan King, 11 Noa seru Nakaitaci, 10 David Skrela, 9 Ludovic Radosavljevic, 8 Elvis Vermeulen, 7 Alexandre Lapandry, 6 Julien Bonnaire, 5 Nathan Hines, 4 Julien Pierre, 3 Clement Ric, 2 Benoit Cabello, 1 Raphael Chaume,
Replacements: 16 Ti'i Paulo, 17 Daniel Kotze, 18 Jamie Cudmore, 19 Damien Chouly, 20 Kevin Senio, 21 Brock James, 22 Lee Byrne, 23 Davit Zirakashvili.