Paris - South African scrum-half Rory Kockott kicked 17 points as Castres beat Montpellier 25-12 to reach the French Top 14 semi-finals on Saturday.
Kockott, the second-highest points scorer in the competition this season behind Toulon's former England star Jonny Wilkinson, succeeded with five penalties and one conversion.
Castres will now face Clermont on May 25 for a place in the final with Toulon tackling defending champions Toulouse, who beat Racing Metro 33-19 on Friday.
Saturday's result meant that Castres extended the season for joint-coaches Laurent Labit and Laurent Travers, who will take charge of Racing next season.
With the sides level at 9-9 at the interval, Castres put daylight between themselves and Montpellier in the 47th minute when back-row forward Antonie Claassen scored the game's only try.
Remi Tales also knocked over a 35-metre drop goal to put Castres into a second semi-final in three years.
Benoit Paillaugue kicked all of Montpellier's points.
His team's afternoon went from bad to worse when lock forward Mamuka Gorgodze was red-carded 10 minutes from the end for stamping.
Gorgodze had earlier been sin-binned and while he was off the pitch, Castres took the score from 9-9 to 19-9.
"Their numerical inferiority cost them dear. We went into the ascendancy at that time," said Labit.
"We know that we cannot compete with our semi-final opponents over a 26-game season, but perhaps in a one-off match we can do enough to worry them."
Montpellier skipper Fulgence Ouedraogo summed up his team's miserable afternoon.
"It was a long journey into hell. We gave away too many penalties, too many fouls," he admitted.