Racing beat 14-man Stade Francais 33-20 in a Top 14 play-off to set up a semi-final against Toulouse in Spain next week.
European champions La Rochelle will play the winners of the second play-off between Lyon and Bordeaux-Begles, who meet on Sunday.
Scotland fly-half Finn Russell booted four penalties to help Racing past Parisian rivals Stade, with tries coming from Juan Imhoff and Wenceslas Lauret.
Stade suffered a massive blow when flanker Marcos Kremer was red-carded in the fifth minute after his shoulder made contact with Russell's head at a ruck.
Racing immediately turned the screw and the pressure told when, five minutes later, Kremer's Argentina teammate Juan Imhoff darted over for the opening try.
Antoine Gibert, on as a replacement for Russell, hit the conversion.
Stade pulled back a penalty through Joris Segonds, but Racing responded with their second try thanks to Wenceslas Lauret, the flanker forcing his way over from close range.
Gibert again converted and Russell, back on after passing head protocol tests, booted a penalty to stretch Racing's lead to 17-3.
Under a blazing Parisian sun, Stade No 8 Sekou Macalou drove over for a deserved try to hand the side a glimmer of hope at their Stade Jean-Bouin home, Segonds converting.
The momentum swung further when Racing hooker Camille Chat saw yellow for collapsing a maul.
Stade saw a chink and opted for a series of scrums that saw Racing reduced to 13 after Eddy Ben Arous was also dispatched to the sin bin.
One more collapsed scrum left referee Pierre Brousset with no choice but to award Stade a penalty try to leave the two sides tied at 17 points apiece at half-time.
Russell, who will leave Racing for English Premiership club Bath after this year's Rugby World Cup, kicked three penalties in the first 15 minutes of the second period to restore his side's lead.
Segonds ensured the final 10 minutes would be played on a knife edge as he put over a penalty to bring Stade within six points.
Lauret snaffled a turnover that handed Racing a crucial penalty with Stade pressing.
Russell cleared the lines, but as Stade tried to counter from deep in their half, the ball was spilled in midfield and Gael Fickou gathered to cross for a soft try, the Scotsman converting for the final word.
Also this weekend, in the promotion playoff later on Saturday, Grenoble, who lost in the second-tier ProD2 final last weekend, host Perpignan, who finished in 13th place in the top-flight, to a sold-out Stade des Alpes.
lp/gj
