Quins have unhappy memories of April's quarter-final loss to Munster at the Twickenham Stoop but the Scarlets haven't reached the knockout stages for seven seasons. The Welsh region were semi-finalists in 2000, 2002 (as Llanelli) and 2007 but are substantial underdogs to match that feat this time around given what lies ahead in the Pool.
Their English opponents have claimed European glory on three occasions in the Amlin Challenge Cup and Shield and won their domestic crown in 2012, although they have never been past the last eight in the continent's top competition.
Quins are yet to win at home this season, having been beaten by Northampton and Saracens at The Stoop and they head into Saturday's fixture on the back of a disappointing defeat at London Irish last time out. Conor O'Shea's men have beaten both Worcester and London Wasps in the league, though, and have picked up three bonus points from their first five games.
It's been a similar story for the Scarlets, with Simon Easterby's side also winning two and losing three at the start of the season. They did win on the road at Edinburgh in Round 4 of the PRO12 but were edged out by league leaders Glasgow on home soil last weekend.
Scarlets, Wales and Lions centre Jonathan Davies says they will travel in high spirits, though, and he will be hoping they can draw on their stunning opening round win at Northampton two seasons ago as they go in search of an upset.
"Hopefully we can draw on our experiences from games like that and use it as positively as we can," said Davies. "People wrote us off completely in that game but we went up there and got a bonus point win.
"The most important thing is to go up there with the attitude of doing a job and concentrating on your own performance. We've done it before and I'm sure we can do it again this year."
Saturday, 12 October 2013; 15:40, Twickenham Stoop; Sky Sports
Referee: John Lacey (Ire)
Assistant Ref: Peter Fitzgibbon (Ire) Assistant Ref: Olly Hodges (Ire)
TMO: Marshall Kilgore (Ire) Citing Commissioner: Jean-Claude Legendre (Fra)
- These two clubs met in the pool stage of the 2008/09 tournament, with Harlequins claiming two narrow victories (29-22 and 29-24).
- Overall, Quins have six wins, two draws and seven losses against Welsh opposition in the Heineken Cup.
- Scarlets have lost their last three and five of their last six matches against English clubs in the tournament.
- The Welsh outfit lost all six of their matches in the pool stage of last season's tournament.