Priestland was handed the number 15 shirt for Wales' opening warm-up appointment with England.
Fate, though, then played its part when regular number 10 Stephen Jones suffered a calf injury during final preparations at Twickenham and Priestland was told he would be moving positions, six minutes before kick-off.
And the 24-year-old Scarlets player has not looked back since, emerging as first choice fly-half despite the presence of players like Jones and James Hook in Wales' World Cup squad.
"I was in a bit of a panic about remembering the calls," said Priestland of his Twickenham experience.
"That day the players around me, especially Mike Phillips and Jamie Roberts, helped me. It was probably five or six minutes before kick-off. My first thought? I can't really say it now.
"All summer the coaches asked me to run at full-back because they said at the World Cup you need players who can play in more than one position.
"I have always thought I was a 10. I never really thought I was a full-back, but I learnt last year and the year before that you can't say no to any position. I was just happy to play."
Priestland now finds himself in the key playmaker role, driving Wales towards a possible first World Cup semi-final appearance for 24 years, although he admits there are times when the whole experience has not sunk in.
"It is still a bit surreal to turn up here and see all these microphones," he added. "I don't know what to make of it.
"But I'm delighted to be here and enjoying it. Hopefully, it lasts.
"I get a bit nervous on the day of the game. I never like the build up, especially the evening kick-offs as you don't really know what to do with yourself.
"But I have surprised myself with how relaxed I am, and I think that is a lot to do with the training we've had and the fact that we have a lot of confidence in each other."
Priestland's individual battle with Ireland fly-half Ronan O'Gara promises to have a major bearing on which team progresses into a last-four meeting with England or France.
And Priestland has total respect for one of Ireland's finest international players.
"O'Gara is probably one of the best kickers of the ball in the game," he said.
"I have played against him a few times for the Scarlets against Munster and I have always been impressed with him, just the way he organises the game and organises the forwards.
"He gives Ireland a lot when he plays. We are going to have to try to manage him and make sure he doesn't keep putting Ireland in the corners and putting us under pressure.
"Usually, I've been at the wrong end of the result when I have played against him. Hopefully, it will be different this weekend."
Priestland will be entrusted with goalkicking duties on Saturday, and also accepts that drop-goals could prove critical, especially if the contest is as tight as most pundits expect.
He missed a late drop-goal when Wales lost 17-16 to world champions South Africa four weeks ago.
"Hopefully, my (drop-goal) record goes from nil in three to one in four," he added. "Hopefully, if it does come down to a drop-goal, I won't make such a mess of it.
"Last week in training, (prop) Adam Jones was going off the field and dropped one from 40 metres. I told him he couldn't do any more like that.
"The coaches have said all summer just to play with our heads up. We have to get a good balance, but if it is on to run from deep in our own half then we have to back ourselves.
"It's something they will reinforce this week, to back ourselves and not go into our shells."
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