Despite the front-row rearrangment the Scarlets found a rhythm and they edged ahead with a well-struck penalty by Ceiron Thomas in the wake of a Steve Jones challenge on Regan King.
The Borders had a great opportunity to seize the initiative as Jones sent the visiting defence into a panic with a beautifully weighted chip. Yet this opportunity was lost when Calum MacRae spilled the ball virtually underneath the crossbar.
MacRae made amends by slotting over a penalty, which inspired the Scots to create a top-notch touchdown. Jones snapped up a loose ball inside his own half before freeing Garry Law. He acted as the perfect link to give Danielli the time and space to gallop to the target from 45 metres out. MacRae added the extra points, but the joy was diluted when Danielli was taken off injured and was replaced by Bryan Rennie.
The stunned Scarlets regrouped before the break and they narrowed the difference with a score by Jon Mills at the end of a forward surge. Thomas converted. However, there was still time for MacRae to land his second penalty to snatch back the lead for the home players.
Llanelli had the better of the exchanges after the restart and Thomas was on target with a 50th minute penalty to tie the scores and set up a tense finale. He struck again midway through the half to give the Welshmen the edge - and repeated the feat in the dying seconds of the game to make sure of the Scarlets' success.
For Border Reivers:
Penalties: MacRae 2
For Llanelli Scarlets:
Penalties: C.Thomas 4
Border Reivers: 15 Calum MacRae, 14 Simon Danielli, 13 Ben MacDougall, 12 Garry Law, 11 Brendon Daniel, 10 Steve Jones, 9 Brendan McKerchar, 8 Scott Gray, 7 Andy Miller, 6 Scott Newlands, 5 Opeta Palepoi, 4 Craig Hamilton, 3 Bruce Douglas, 2 Steve Scott, 1 Ryan Grant
Replacements: 16 Sean Crombie, 17 Ed Kalman, 18 Richie Vernon, 19 John Dalziel, 20 Rob Chrystie, 21 Gregor Townsend, 22 Bryan Renni
Llanelli Scarlets: 15 Barry Davies, 14 Dafydd James, 13 Regan King, 12 Matthew Watkins, 11 Darren Daniel, 10 Ceiron Thomas, 9 Clive Stuart-Smith, 8 Nathan Thomas, 7 James Bater (c), 6 Jonathan Mills, 5 Adam Jones, 4 Inoke Afeaki, 3 Craig Dunlea, 2 Aled Gravelle, 1 Iestyn Thomas
Replacements: 16 Deacon Manu, 17 Ken Owens, 18 Lou Reed, 19 Vernon Cooper, 20 Liam Davies, 21 Morgan Stoddart, 22 Garan Evans