Harlequins lost in Ireland but won in the Heineken Cup quarter-final qualifying stakes a day later when the Scarlets pushed them into an unassailable Pool 4 lead and a place in the last eight for the first time since 1998.
The 31-17 Scarlets win over Stade Français Paris ended the French club's hopes; provided the Welsh club with the victory to end a run of 11 successive tournament defeats and propelled Quins into the knock-out stages.
Ironically, the Scarlets now travel to the Twickenham Stoop to clash with the team for whom they did a huge favour with Round 5 Heineken man of the match Stephen Jones warning Quins they may be out but they are far from down.
"Although it is now over for us in this season's Heineken Cup, our ambitions are to keep going forward as a side, we need to keep on gathering momentum and improving our performances," said the Wales fly half.
"We have a tough game against Harlequins before refocusing on the Celtic competition, and, while it's all about performing well now, it is also important to secure points for the ERC European Rankings for next year.
"As for the match on Saturday in London, when you see the Scarlets play the type of game that we played against Stade today - and which we know we can do and have done in the past, though probably didn't do last year - I think we have the ability to win.
"Going back to our first game against the Quins earlier in the tournament we were outstanding in the first half and probably should have won a bonus point by half-time.
"But we didn't achieve that and the second half really knocked us and it took us a long time to recover so hopefully we'll see a full 80 minute performance this time.
"We did Quins a favour by beating Stade but it was always about us in that game and we would like to think we've re-established some European credentials."
It was the Scarlets' debut European win at their new home at Parc y Scarlets and Jones believes it will prove only the first of many.
"We seem to have built up something nice here in this stadium and it feels like home now," said the player who contributed 16 points with the boot to the victory over Stade to take his Heineken Cup tally to 748.
"Winning here was definitely good for us, good for the fans and good for the future. We have to make sure people are keen to come here and if we keep playing like that there's no reason why we shouldn't continue winning.
"We've not given our fans much to cheer about in the last 18 months so we are pleased for them with this important result.
"From our perspective, things don't happen straight away so it's with moments like this victory in the Heineken Cup against a top team that you create more history for the club.
"Because not only are we are very happy with the victory, but also our performance was good. The challenge for us was to sustain the pressure for a full 80 minutes and we managed that.
"We probably showed more enthusiasm and hunger than them, the boys believed in themselves and were outstanding."