Scarlets threw the battle for Heineken Cup Pool Four wide open on Saturday with a surprise 33-19 win over Harlequins.
Scarlets put the embarrassment of last season's Heineken Cup whitewash behind them as backs Rhodri Williams, Scott Williams and Jordan Williams all scored outstanding tries for the visitors, who led from start to finish.
Mike Brown touched down twice for the English club, what it wasn't enough as the hosts were forced to settle for a losing bonus point despite dominating the closing stages.
The defeat was Harlequins' fourth loss in six matches across all competitions this season.
Rhys Priestland contributed with an excellent kicking performance under pressure, adding three conversions and four penalties with the Wales fly-half also setting up the opening try for R.Williams.
The livewire scrum-half collected Priestland's inside pass after the Scarlets 10 combined with Kristian Phillips down the right touchline to unlock the Harlequins defence within the opening minutes. Priestland's penalty a few minutes later had the visitors comfortably ahead at 10-0.
Priestland and Evans traded penalties as Harlequins tried their hardest to establish a foothold in the match, reeling after defeat away to London Irish last weekend.
Brown, who impressed throughout as he vies for England's full-back spot ahead of the November Internationals, was sharp enough to cut inside for the hosts' first try after a favourable bounce of the ball for Nick Easter enabled to find Brown in space.
With the damage only limited to three points, the home faithful at The Stoop gradually found their voice until Scott Williams silenced them with a wonderful try.
The Wales centre burst through and fixed England scrum-half Danny Care with a dummy, before beating Matt Hopper for pace to touch down under the posts. It left the Scarlets 20-10 ahead at the break.
Samson Lee's sin-binning for a stamp on the face Care was only given a yellow card but could have been red, a key moment that played out in the visitors' favour.
Nick Evans added a drop goal to bring the hosts back within seven points but Priestland struck twice with difficult penalty attempts as the West Wales outfit impressively controlled the tempo through their scrummaging and choke tackles in defence.
Quins piled on the pressure and Evans took the two chances that came his way to put the result in the balance heading into the final quarter.
Scarlets were not done, though. Scott Williams brilliantly stripped Joe Marker of the ball on the Quins 10-metre line to gain possession, and the three-time European semi-finalists spread the ball wide to left wing Jordan Williams who bumped off three defenders and cut inside Matt Hopper for a sublime opportunist score. Priestland converted to make it 19-33.
Brown demonstrated his finishing instincts once again with a well-taken score and Evans converted to put Quins back within one score of Simon Easterby's men.
With Rob Evans in the sin-bin for repeated team infringements Scarlets faced a nervy final 10 minutes, but they held on to give them their best win in Europe since defeating Northampton at Franklin's Gardens two years ago.
Tries: Brown 2
Cons: Evans 2
Pens: Evans 3
Tries: R. Williams, S. Williams, J.Williams
Cons: Priestland 3
Pens: Priestland 4
Yellow cards: Lee, Evans
Harlequins: 15 Mike Brown, 14 Tom Williams, 13 Matt Hopper, 12 Ben Botica, 11 Sam Smith, 10 Nick Evans, 9 Danny Care, 8 Nick Easter, 7 Chris Robshaw (c), 6 Maurie Fa'asavalu, 5 George Robson, 4 George Merrick, 3 Will Collier, 2 Rob Buchanan, 1 Joe Marler.
Replacements: 16 Dave Ward, 17 Darryl Marfo, 18 Kyle Sinckler, 19 Tom Guest, 20 Luke Wallace, 21 Karl Dickson, 22 Paul Sackey, 23 Charlie Walker.
Scarlets: 15 Liam Williams, 14 Kristian Phillips, 13 Jonathan Davies (capt), 12 Scott Williams, 11 Jordan Williams, 10 Rhys Priestland, 9 Rhodri Williams, 8 Josh Turnbull, 7 John Barclay, 6 Aaron Shingler, 5 George Earle, 4 Jake Ball, 3 Samson Lee, 2 Emyr Phillips, 1 Phil John.
Replacements: 16 Kirby Myhill, 17 Rob Evans, 18 Jacobie Adriaanse, 19 Joe Snyman, 20 Craig Price, 21 Gareth Davies, 22 Steven Shingler, 23 Gareth Maule.
Venue: Twickenham Stoop
Referee: John Lacey (Ireland)
Assistant referees: Peter Fitzgibbon (Ireland), Olly Hodges (Ireland)
Television match official: Marshall Kilgore (Ireland)