Defending champions Castres will play European champions Toulon in a repeat of the 2013 French championship final after they edged Montpellier 22-19 in an engrossing extra-time encounter in Lille on Saturday.
For Castres it was the third successive time they had beaten Fabien Galthie's Montpellier in the play-offs after their opponents had got the better of them on their way to the final in 2011.
While Castres South African scrum-half Rory Kockott scored 14 of their points it was 35-year-old Fijian centre Seremaia Bai who dropped a rare goal on the stroke of half-time of extra-time that made the difference in the end.
Castres will meet Toulon at the Stade de France in a fortnight, a year on from beating them 19-14 in the final, and just a week after their opponents play English side Saracens in the European final.
Toulon beat Racing-Metro in Friday's first semi-final.
"It was a really tough, hard match as they always are with Montpellier," said Castres captain and fly-half Remi Tales.
"However, the important thing is we are in the final having held our nerve and we have Act II of last year's final," added the 30-year-old France international.
Montpellier fullback Pierre Berard was distraught.
"It is difficult to take," said the 22-year-old.
"We wanted to and should have played a lot better than this. We were too tense and nervous."
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