10 Dec, 12:23
Ireland's John Lacey will referee his first ever RBS 6 Nations match in February, while Alain Rolland is also included in the Elite Panel in what is his last season.
Simon Easterby's side, who had tallied up 10 tries in their previous two RaboDirect PRO12 outings, were held scoreless in a first half that ended 8-0 in Connacht's favour.
Miah Nikora, excelling at out-half with the injured Dan Parks watching on, had kicked a penalty and set up Eoin Griffin for a 39th minute try.
But the Scarlets rediscovered their scoring touch as Williams and George North cut through for three tries in a superb 11-minute spell.
Nikora's second penalty reduced the margin to 17-11, however the Scarlets garnered a late bonus point when stand-in captain Jonathan Davies ran in an intercept score.
Rhys Priestland flung an early penalty wide at the Sportsground, the Welsh region benefiting from the wind advantage and a couple of charges from Samson Lee and North.
Connacht grew into the game with Ethienne Reynecke and Andrew Browne standing out in the forwards, and David McSharry putting in some powerful hits in midfield. Nikora then opened the scoring, punished a scrum offence by Lee with a 10th minute penalty.
A Willie Faloon-inspired surge almost led to a first try for the hosts, however captain Gavin Duffy spilt Tiernan O'Halloran's pass close to the right corner.
The Scarlets were making little headway until a lineout poach by Richard Kelly set their young backs in motion - North was put into space on the right before Nick Reynolds threatened on the opposite wing.
Handling errors prevented both sides from building momentum and too often the Scarlets fell foul of referee Neil Paterson at the breakdown, most frustratingly after an opportunist break from Josh Turnbull.
Starved of territory, Connacht played a smart game in defence and a series of deft passes unleashed centre Griffin for a near miss on the left.
But the former Ireland Under-20 international managed to score in the opposite corner just moments later, a low slung pass from Nikora taking three Scarlets players out of the game and putting Griffin over out wide.
Nikora was unable to add the difficult conversion and the interval came at just the right time for the Scarlets, who dominated on the resumption.
North burst through to create an overlap and pass for young full-back Williams to slip over for his first try which Priestland converted.
Priestland missed a penalty but the visitors had found their groove in attack, with North brushing past Duffy's attempted tackle on the right to raid in for his five-pointer.
Another missed tackle cost Connacht dearly as Williams evaded Griffin's grasp to score again to the left of the posts, extending the lead to 17-8.
Connacht responded with a penalty from Nikora on the hour mark, and they pushed hard for a match winner until they were dealt a cruel blow right at the death.
In the very last minute, Davies intercepted a high pass from Nikora in the Scarlets 22 to embark on a long range gallop to the try-line which sealed a bonus point for the visitors and ripped one away from Connacht.
|Connacht Rugby Score Card|
|Scarlets Score Card|