New home coach Steve Bates had emphasised the fact that his revamped squad could not have been handed a harder task to start their campaign.
And the first hour of the match followed the form-book, as the champions dominated. But the Borders refused to buckle and looked the stronger in the closing stages.
The contest appeared to be over at the break, by which time the Welshmen had carved out three tries, all converted by Gareth Bowen.
In the opening session, Borders had earned a chance to break the deadlock when the Scarlets wandered offside.
Kiwi stand-off Charlie Hore, making his competitive debut for the team, connected crisply - but the ball crashed back from the post.
Llanelli made the most of their reprieve by surging up field to seize the initiative. Bowen sent a pinpoint chip into space, giving left-wing Selesi Finau plenty of time and space to scoop it up and score in the corner.
And they left the Borders foundering with two more touchdowns. First, former Test lock Chris Wyatt was driven over by his powerful pack colleagues - then flanker Gavin Thomas was in the right place at the right time to finish off a sweeping handling move.
Borders came out at the restart with more purpose, but there was more frustration as winger Nikki Walker failed to latch on to an interception.
They eventually got off the mark in the 51st minute when ex-Glasgow star Stuart Moffat pounced on a loose ball to dash 35 metres for the try.
Hore then found his touch with the boot to slot a penalty - but any prospect of a comeback was snuffed out as Simon Easterby powered through for the fourth Llanelli score.
Borders, however, had the last word when Chris Cusiter sneaked in for their second touchdown.
: Tries: Moffat, Cusiter. Cons: Hore. Pens: Hore.
: Morton, Moffat, MacRae, Dickson, Walker, Hore, Cusiter, Thomson, Scott, Douglas, MacLeod, Stewart, Gray, Blair, McEntee.
: Tries: Finau, Wyatt, G. Thomas, Easterby. Cons: Bowen 3.
: Evans, Havili, Selley, Watkins, Finau, Bowen, Phillips, I. Thomas, Rees, Thiel, Griffiths, Wyatt, Easterby, Quinnell, G. Thomas.
: S McDowell (Ireland).