The sides will battle it out at Carmarthen Park for the right to face SWALEC Cup champions, and reigning league champions, Pontypridd in the final on May 18.
Both Quins and Keys have the chance to capture their first ever Premiership title having had their best league seasons to date.
The West Walians will be looking to build on their second-place finish with victory over the Cup finalists after finishing eight points clear of their rivals. Quins have had their finest campaign since their inception, winning 18 of their 22 games and picking up 11 try-scoring bonus points along the way.
But Keys will be buoyed by their late charge into the play-off picture. They edged out Llandovery who ended on the same amount of points, but the Gwent side crucially bagged one more win to send them into the final three.
Carmarthen go into the semi-final in scintillating form having won all of their last five league games, including an away victory against their play-off opponents.
Gregg Woods’ Keys have won three of their last five outings having put together a three-game winning run with triumphs over Llandovery, Bedwas and Newport before slumping to a final-day defeat against Bridgend Ravens.
They will look to bounce back from Millennium Stadium heartbreak after going down 21-8 to Pontypridd at the SWALEC Finals Day a week ago but they will have to do it against a team that have done the league double over them this year.
A 19-point haul from Gloucester-bound fly half Aled Thomas led the Quins to victory in the first game between the sides this season and they completed the double over Leon Andrews’ men with a hard-fought 7-3 win at Pandy Park after Rhys Williams bagged a 60th minute try.
Carmarthen Quins: 15 Aled Thomas 14 Gavin Thomas 13 Iolo Evans 12 Rhys Williams 11 Chris Banfield 10 Dan Jones 9 Kieran Hardy 8 Rory Gallagher 7 Ellis Lloyd 6 Sion Bennett 5 Haydn Pugh 4 Carwyn Jones 3 Ian Jones 2 Rhodri Clancy 1 Gareth Thomas
Replacements: 16 Ryan Elias 17 Gethin Robinson 18 Javan Sebastian 19 Aidan Laxton 20 Luke Joseph 21 Huw Rees 22 Howard Thomas 23 Morgan Griffiths
Cross Keys: 15 Ryan Lee 14 Elliott Jones 13 Leon Andrews (c) 12 Phil Williams 11 Nathan Trowbridge 10 Dorian Jones 9 Owen Jones; 1 Richard Cornock 2 Darren Hughes 3 Anthony Lott 4 Ollie Hodge 5 Tom Lampard 6 Ieuan Jones 7 Jevon Groves 8 Adam Powell
Replacements: Henry Palmer, Craig Gould, Danny Hodge, Sean Higgins, Jack Perkins, Nathan Williams, AN Other.
