Captain Ken Owens scored the only try of a scrappy match to help Scarlets register a crucial Guinness Pro12 derby victory over Cardiff Blues at Parc y Scarlets.
The win lifted the west Wales region into sixth place in the league to boost their chances of claiming the final automatic qualification spot for the European Champions' Cup.
It now goes down to final match of the regular season at the weekend when Scarlets travel to Treviso, while Connacht in seventh, four points behind, will host leaders Ospreys.
Owens' try came in the second half after an error-strewn match had been neatly poised at 9-6.
Scarlets were indebted to 11 points from the boot of Rhys Priestland, while Blues' points came by way of two penalties from Rhys Patchell.
Scarlets were at near full-strength although they were without Regan King with concussion. This was Priestland's final home game before he joins Bath in the summer.
The west Wales region were unchanged from the team that beat the Dragons 29-17 on Judgement Day.
Cardiff Blues centre Dafydd Hewitt has been forced to retire from rugby immediately due to a neck injury.
The 29-year-old has only been able to make two appearances this season due to injury, and says being forced to retire is "the worst feeling".
The former Blues vice-captain has played 122 times for the region, scoring 13 tries, since 2006. He previously played for Llantwit Major and Cardiff RFC.
Hewitt was part of the Wales Under 21 Grand Slam-winning team in 2005, and captained the Under 21s at the World Cup in 2006. He was part of the Wales squad that won the Rugby World Cup Sevens in 2009.
It is hard to put into words how I am feeling at the reality of never wearing my Blues jersey again.
No rugby player expects to be told that they can never again play the game they love - it's the worst feeling. It was my childhood dream to play rugby for the Cardiff Blues, the team I followed as a boy and dreamt of representing. I have loved every moment of being a 'Cardiff Blue' and knew early on that it was a club I never wanted to leave.
I'd like to thank everyone at the Blues; the coaching staff, the management, and the medical team for their invaluable support, especially during the past few months.
I feel privileged to have played with so many great players and have forged friendships for life. I will miss everything about playing rugby and will always be a Cardiff Blue.
Cardiff Blues Chief Executive Richard Holland described Hewitt as "an outstanding servant, a true role model for young aspiring players, a hugely popular and much-admired professional."
Cardiff Blues lost to Ulster tonight in the Guiness Pro 12.
Despite having no points at half time the Blues fought back, but couldn't do enough to close the gap on the hosts at the Kingspan Stadium.
The final score was Ulster 36 - Cardiff Blues 17.
Treviso beat Cardiff Blues 40 - 24 in the PRO12 in Italy.
Adam Thomas, Josh Navidi and Lloyd Williams each scored tries with Rhys Patchell making up the rest of the team's points.
FT Benetton Treviso 40 Cardiff Blues 24
Cardiff Blues have lost 33-16 to Munster in the PRO12 in Cork.
Flanker Josh Navidi got the Welsh side's try in the second half, and fly-half Rhys Patchell kicked the rest of the Blues' points.
The result sees Munster move to the top of the PRO12 table.
HT Munster 9 Blues 9
Dacey with the initial carry and then Navidi picked up and refused to be stopped! Now 23-16 to Munster with nine minutes left...
Former Cardiff Blues captain Andries Pretorius has been forced to retire from the sport after being diagnosed with a muscle disorder.Read the full story ›
Cardiff Blues pulled away from London Irish with two second-half tries in their European Challenge Cup clash.
Scrum half Lloyd Williams touched down in the corner just after the hour, after Cory Allen was held up just short of the line.
Substitute hooker Kristian Dacey then crossed the line after relentless pressure from the Blues forwards.
The visitors scored a late penalty try of their own, to make the final score at Cardiff Arms Park 24-14.
The Blues have now won all three of their games in the European Challenge Cup.
The Blues were neck and neck with London Irish at half time in the European Challenge Cup.
Shane Geraghty crossed for Irish after 8 minutes, only for Richard Smith to pull one back for the Blues. New arrival Gareth Anscombe slotted the conversion, but missed a penalty later.
Guinness PRO12 leaders Ospreys made it six wins from six by beating Welsh rivals Cardiff Blues 26-15 in a one-sided clash at the Liberty Stadium.
The match may have finished two tries each, but the Ospreys dominated possession and territory for large periods and could easily have achieved a bonus-point win.
The home side scored a try in each half for Dan Evans and Rhys Webb, with Dan Biggar supplying 16 points with four penalties and two conversions. The Blues rallied at the end with tries for Lloyd Williams and Kristian Dacey.
Adam Jones faced his former region Ospreys for the first time since joining them in August.
In the early stages the Blues struggled to get their hands on the ball and soon found themselves 9-0 behind after 11 minutes through three Biggar penalties.
It did not get any better for the Blues as Warburton found himself yellow-carded for not being 10 metres when Webb attempted a tap penalty.
That offence saw Biggar make it four penalties on the trot.
The Blues did get on the scoreboard on 26 minutes with a finely struck penalty from Rhys Patchell on halfway.
The only thing missing from the Ospreys' confident performance was a try, but that arrived on 29 minutes in the form of full-back Evans went over after another wave of attacks by the home side.
In the dying minutes the Blues finally got some momentum and scored two late consolation tries.
Leinster were made to fight all the way for their second Guinness Pro 12 win at the RDS Arena with a late bonus point try sealing a 37-23 triumph over the Cardiff Blues.
Rhys Ruddock's storming solo try had Leinster leading 13-9 at half-time, with Ian Madigan kicking eight points and Rhys Patchell contributing all 9 for Cardiff.
Converted efforts from Cory Allen and Sam Warburton put the Blues back in the hunt at 27-23, but the defending champions made certain of victory with Madigan's third penalty and Luke McGrath's late try.