Clermont Auvergne overcame Flip van der Merwe's red card to ease to a 37-31 win over defending champion Racing 92 in the Top 14 semi-final, setting up a showpiece with Toulon.

Saturday's (AEST) last-four clash had seen Toulon mount a comeback and knock out La Rochelle, which had led but saw the game turn when Pierre Aguillon was dismissed.

It looked as if Clermont was set to suffer the same fate as La Rochelle at Stade Velodrome when Van der Merwe was sent off for a high tackle on Teddy Thomas with his side ahead 19-6 a minute into the second half.

But instead of folding, Clermont produced a stirring second-half showing, inspirational fly-half Camille Lopez scoring the second of his double while half-back partner Morgan Parra, who accounted for 17 of his side's points, did the damage with the boot.

Fritz Lee went over for Clermont's fourth try as it ended Racing's defence despite a late flurry from the holder to book a meeting with Toulon at the Stade de France next weekend.

Dan Carter's early penalty was quickly cancelled out by a three-pointer from Parra, and Clermont then moved into the lead as Damian Penaud dived over in the left-hand corner at the end of a slick passing move.

Carter cut the gap to two points with the boot but Racing had a mountain to climb at the break after Lopez crossed out wide on the right and Parra made it a 13-point gap by splitting the uprights twice more just before half-time.

The game looked to have taken a sharp turn in Racing's favour when Van der Merwe was dismissed after a consultation with the TMO and Carter quickly added two penalties to fuel hope of a comeback.

But Lopez caught the Racing defence napping to scoot over from the ruck for his second, Parra making no mistake with the boot.

Racing's Chris Masoe was then sin-binned for tackling in the air and Parra knocked over another six points in his absence.

By that point desperation had set in for Racing and a stray pass was kicked ahead by Clermont full-back Nick Abendanon, with Lee dotting down a couple of phases later to put the game firmly beyond doubt.

Masoe somewhat atoned for his earlier infraction with a late double and Ben Tameifuna powered over late on to give the scoreline a more flattering look on a disappointing day for Racing.