Castres went into the break for European action with a much-needed 34-8 bonus-point win over Racing 92 at Stade Pierre Antoine on Sunday.
The hosts scored five tries in all as a second-half onslaught saw them see off the challenge from the Parisians, who remain in third place.
Castres began the game in sprightly fashion and took the lead after just seven minutes, breaking the Racing 92 defence early with the try going the way of flank Alexandre Bias. France scrum-half Rory Kockott slotted the extra two points to give them a handy 7-0 cushion.
Racing though fought back and the only other points of a low-scoring first-half went the way of Johan Goosen, whose penalty on 14 minutes meant the deficit was cut to four as both teams went off for the break. Racing were looking to avoid just their second loss of the season.
Defeat seemed very much on the cards for the Paris outfit soon after the turnaround as number eight Alex Tulou crossed for Castres in the 43rd and 52nd minute to make it 22-3, with the result looking beyond doubt even with 28 minutes to run at the Stade Pierre Antoine.
Former Toulon finisher David Smith also went over before the hour for Castres' fourth try but Racing would hit back moments later as Louis Dupichot's score gave his side a glimmer of hope at 29-8 for what would have been a miraculous comeback from the visitors.
But Castres would close out the game with ease with their fifth try arriving via Remy Grosso on 68 minutes, as they pick up a valuable win that takes them tenth in the standings, entering the European Challenge Cup in confident mood.
Tries: Bias, Tulou 2, Smith, Grosso
Con: Kockott 3
For Racing 92:
Castres: 15 Julien Dumora, 14 David Smith, 13 Rudi Wulf, 12 Remi Lamerat, 11 Remy Grosso, 10 Daniel Kirkpatrick, 9 Rory Kockott, 8 Alex Tulou, 7 Piula Faasalele, 6 Alexandre Bias, 5 Rodrigo Capo Ortega, 4 Richie Gray, 3 Yohan Montes, 2 Brice Mach, 1 Mihai Lazar.
Replacements: 16 Marc-Antoine Rallier, 17 Lucas Martinez, 18 Christophe Samson, 19 Sitiveni Sivivatu, 20 Julien Seron, 21 Romain Martial, 22 Yannick Caballero, 23 Saimone Taumoepeau.
Racing 92: 15 Brice Dulin, 14 Joe Rokocoko, 13 Etienne Dussartre, 12 Alexandre Dumoulin, 11 Louis Dupichot, 10 Johan Goosen, 9 Xavier Chauveau, 8 Antonie Claassen, 7 Thibault Dubarry, 6 Yannick Nyanga, 5 Ross Filipo, 4 Juandre Kruger, 3 Ben Tameifuna, 2 Dimitri Szarzewski, 1 Eddy Ben Arous.
Replacements: 16 Virgile Lacombe, 17 Davit Khinchagashvili, 18 Francois van der Merwe, 19 Luc Barba, 20 Mike Phillips, 21 Benjamin Dambielle, 22 Henry Chavancy, 23 Cedate Gomes Sa.
Referee: Romain Poite