The home side’s victory at Llantwit Fardre leaves them with 26 points whilst a losing bonus point for the Scarlets means they are a single point ahead of the Ospreys, who travel to Sardis Road next week to face the Blues North.
A victory for the Ospreys will see them into the March 5th final, at Pontypridd, whilst a losing bonus point means try count would determine whether the Ospreys or Scarlets would go through to meet the North.
Tries from flanker David Barry, centre Sam Campbell and fullback Josh Wills, plus two conversions and a penalty from outside half Joseph Griffin put the hosts into a commanding 22-6 half-time lead.
However, tries from centres Dale Ford and Luke Williams put the Scarlets within six points as the game moved into its final quarter.
Griffin kicked a penalty to make it 25-16 and that, in the end was enough, despite two further penalties from the Scarlets in the closing stages.
In the other game, the Dragons led Blues South 8-7 at half-time thanks to a try after 24 minutes from Matthew Jolly and a penalty from Lewis Roblin.
Fred White scored a converted try for the South in the first period and after the interval, they ran away with maximum points courtesy of tries from wing Aled Williams, prop Joe Tomlinson and fullback Dan Jones.
Gareth Evans struck back with a try for the Dragons in the 70th minute, but by then the South had done enough to secure another excellent victory.
