But back came the Biancoverde with a passion not seen anywhere in the first half.
First replacement scrum-half Fabio Semenzato went over from a yard out and then No. 8 Robert Barbieri followed suit before Di Bernardo's moment came from a Scarlets infringement after time was up.
Franco Smith's side started slowly in front of a vocal home crowd and allowed Scarlets to take all the initiative.
Priestland hit a drop goal on 13 minutes but it took until the 26th minute for the Scarlets to bring one of their raids into Treviso's 22 to a five-point conclusion and it was the increasingly reliable wing Andy Fenby who crashed over.
Priestland spread the play to the left and Fenby took on Valerio Bernabo and went right through the lock's clutches, carrying two more defenders over the line with him.
But Treviso responded immediately by putting the squeeze on the Scarlets' 22-year-old scrum-half Gareth Davies.
From a lineout five metres from the Welsh region's line, Italy international flanker Alessandro Zanni charged down Davies' box kick and the ball fell perfectly for hooker Leonardo Ghiraldini to crash over.
However Kris Burton failed with his conversion attempt and just before half-time the fly-half was partly at fault for Fenby's second try.
Fielding the ball at full-back, Burton looked for a scissor-switch with Tommaso Iannone but spilt the ball just in front of the halfway line, and Fenby pounced.
Showing equal amounts of football skill and pace, the 26-year-old hacked forward twice and re-claimed the ball to touch down just before it went out of play.
Priestland made no mistake with his first pot at goal in the second half but the home side were finally starting to flex their muscles and set up an outpost in the Scarlets' 22.
The result was a Semenzato try when he sneaked over on 51 minutes and Di Bernardo added the extras to put pressure on the Scarlets for the first time in the game.
Treviso hemmed the Scarlets in to the confines of their own half and their in-your-face defence created another try for Williams. Robert Barbieri charged down a kick on halfway and ran on to side-foot the ball into the goal area and touch down for the game's crucial try.
Di Bernardo converted from out wide to make it a one-point deficit before urging his team onwards with strong directional play. Zanni went the closest to scoring a try but was denied by a matter of centimetres.
But then came the decisive moment, a penalty awarded on the halfway line, dead in line with the posts and Di Bernardo had all the necessary coolness to boot it straight and true to win the game.
|Benetton Treviso Score Card|
|Alberto di Bernardo||2||1||7|
|Scarlets Score Card|