Pontypridd go into the game as favourites, the Sardis Road club having already secured top spot in the Principality Premiership, losing just once so far this season.
It is a record enough to intimidate any team but Carmarthen winger Harries is adamant his side can reach the final on May 4.
Quins have previous for semi-final wins over Pontypridd. In 2010, they ran in six tries at Bridgend's Brewery Field to get the better of Pontypridd and reach the final.
Harries admits both sides have come a long way since that day but is drawing inspiration from that result.
"A lot has changed since then and it's the same for Pontypridd," said the Carmarthen winger.
"Pontypridd had a good result against Cardiff on Tuesday night and we know what's in store. They've only lost one game all season and that's not an easy thing to do.
"There's a big challenge ahead of us on Saturday. It's cup rugby, form goes out the window and it's about who performs best on the day. Hopefully that will be us."
After playing in front of 50,000 people in Hong Kong with Wales Sevens, Harries returns to the Quins side, as does fellow Wales man James Davies, who is on the bench.
Dan Newton starts at fly half with the inform Alex Pickersgill a danger from outside centre.
"We've come off a bad performance against Newport but that's been put to bed now," said Harries.
"We had two tough games against Narberth and Blackwood but now we're excited to play a fellow Premiership side in the semi-final."
If Quins are using memories of 2010 as inspiration, Pontypridd are hoping a repeat result won't be on the cards. Head coach Dale McIntosh has named a strong side and Thomas, who starts at full back tomorrow, has bad memories of that day.
"We went down to the Brewery Field and got hammered really," he admitted.
"Maybe complacency did creep into our game that day but we're an experienced squad now and there are boys who have been here for five or six years. We won't have any complacency this time.
"You may only get one chance to play at the Millennium Stadium but luckily, this Pontypridd squad have played there the last two years. We've got an incentive to get there again this year."
McIntosh makes seven changes to his starting line up from the midweek win at Cardiff. There are welcome returns from injury for winger Chris Clayton and prop Pat Palmer. A late withdrawal from the squad is centre Dafydd Lockyer with a groin strain.
"The last couple of games we've played really well, hopefully we've found a bit of form going into the last part of the season," said Thomas.
"We've got to go into the match confident and hopefully the result will take care of itself."
Pontypridd: 15 Adam Thomas 14 Chris Clayton 13 Gavin Dacey 12 Gary Williams 11 Owen Jenkins 10 Dai Flanagan 9 Gareth James, 1 Chris Phillips 2 Huw Dowden 3 Keiron Jenkins 4 Cory Hill 5 Chris Dicomidis (c) 6 Ed Siggery 7 Rhys Shellard 8 Dan Godfrey.
Replacements: Joel Raikes, Simon Humberstone, Gareth Wyatt, Jack Dando, Jake Thomas, Craig Locke, Pat Palmer, Stuart Williams.
Carmarthen Quins: 15 James Dixon 14 Lee Williams 13 Alex Pickersgill 12 Rheon James 11 Jason Harries 10 Dan Newton 9 Rhod Wells, 1 Gareth Thomas 2 Richard Wilkes 3 Ian Jones (c) 4 Gavin Evans 5 Haydn Pugh 6 Sion Bennett 7 Ellis Lloyd 8 Rory Gallagher.
Replacements: Gethin Robinson, Ricky Guest, Andrew Beaujean, Carwyn Jones, James Davies , Connor Lloyd, Jack Maynard, Darren Daniel.