The Scarlets kept their chances alive in Heineken Cup Pool 4 for another day at least as they stunned big-spending Racing Metro 19-13 at the Stade Yves du Manoir.
Simon Easterby's side could be knocked out when Harlequins host Clermont Auvergne on Saturday afternoon but this win was a huge boost regardless.
A 40-metre solo score from young wing Kristian Phillips just past the hour proved decisive, with Wales fly-half Rhys Priestland adding 14 points courtesy of four penalties and one conversion.
British & Irish Lions stars Jonny Sexton and Mike Phillips stood out for Racing, with the scrum-half grabbing his side's early try, but the Scarlets were good value for a first win in seven games.
The Scarlets started well and they were ahead after four minutes as Priestland made up for an even earlier miss by hitting the target from straight in front.
Racing showed their obvious potential when Phillips initiated a charge down the left that only ended when Argentinian playmaker Juan Martin Hernandez stepped into touch five metres short and they were in front with just nine minutes gone.
Racing may have only scored 14 tries in 16 domestic games but their intentions were clear when Sexton opted to ignore a simple shot at goal in favour of an attacking lineout.
That enterprise brought an immediate reward when Sexton's inside ball to Juan Imhoff put Racing on the front foot and Phillips went over from close range just after Sexton had been hauled down a metre short.
Sexton added the extras from midway between the left touchline and the posts and Racing were 7-3 up.
They looked set to stretch that lead three minutes later as they had the Scarlets in all sorts of trouble but the visitors somehow escaped unscathed.
Virgil Lacombe was denied a try by the efforts of openside John Barclay and two more Scarlets colleagues as they held the hooker up over the line but Racing should still have been celebrating moments later.
Having been on the brink of a penalty try following a brace of quick-fire penalties at the resulting five-metre scrum, Racing were undone as Phil John got underneath former Northampton prop Brian Mujati to win turnover ball and avert the danger.
The rest of the half was much more even, with Priestland cutting the gap to 7-6 with another penalty two minutes before the break and Sexton restoring the four-point gap with the final kick of the half.
Frazier Climo should have scored for the Scarlets when he failed to pick up a bouncing ball with the line at his mercy shortly after the restart, although Priestland did cut the deficit again with a third penalty.
Priestland then put the Scarlets in front with a fourth strike after 55 minutes before Hernandez made it 13-12 on the hour as Sexton recovered from a knock.
But Kristian Phillips then left five Racing defenders for dead to claim a stunning score for the Scarlets two minutes later, displaying devastating pace in providing the most memorable moment of the match.
With Priestland adding the conversion, the Scarlets were 19-13 ahead and they held on for the final 15 minutes with relative ease.