Centre Gavin Evans was Scarlets' hero as his brace helped his side see off the challenge of a physical Newport Gwent-Dragons at Rodney Parade.
Evans' double came in an electric period after the break that saw the Scarlets turn a 16-3 deficit at half-time into a close win in the Magners League showdown.
Dragons coach Paul Turner took a gamble on youth and it nearly paid off with the home side taking the game to the Scarlets and winning the early exchanges.
Turner promoted 19-year-old centre Ashley Smith from the Academy side and scrum-half Alex Walker was making only his second Magners League appearance in the Rodney Parade cauldron.
There were several personal battles between the big men and bragging rights were up for grabs in the all-Wales battle.
In the backrow, the showdown for the Wales No 8 shirt was shaded by Colin Charvis who was immense in defence and led with ball in hand against present incumbent Alix Popham.
Watching Wales coach Gareth Jenkins would have taken note.
It was a tough, physical start as Sweeney and Jones traded penalties early on with the two huge packs squaring up to each other.
But Llanelli soon found themselves a man down when the yellow card was shown to teenage lock Lou Reed for taking a walk on the wrong side of a ruck and committing a professional foul.
The Dragons capitalised when a long Charvis pass led to Nathan Brew darting over for a try converted by Sweeney on the half hour.
A penalty after an indiscreet punch from Scarlets hooker Matthew Rees on opposite number Steve Jones saw Sweeney kick the Dragons into a 13-3 lead before his drop goal on the stroke of half time made it 16-3.
But the Scarlets have far too much experience to capitulate and they hit back with three quick tries immediately after the break to put themselves in front.
First of all centre Evans grabbed his pair as he crossed out wide and was on hand to benefit from a Regan King break. The try was converted by Jones.
To finish the flourish full-back Ceiron Thomas had a simple run in after his backs created the over-lap to capitalise on poor Dragons defence.
Jones missed the conversion, but it was enough for a 20-16 lead after 50 minutes before a Sweeney penalty brought the Dragons back to 20-19.
Sweeney and Jones traded penalties, but the lead stayed in Llanelli's hands.
Dragons: Nathan Brew try, Sweeney con, Sweeney (4) pens, Sweeney drop-goal.
Scarlets: Gavin Evans (2), Ceiron Thomas tries, Jones con, Jones (2) pens.