Coach Warren Gatland has hailed "cool, calm" Rhys Priestland, 24, for seizing the opportunity to be Wales' first-choice World Cup fly-half.
Gatland says a lack of experience was against Priestland before the event and he goes into Saturday's quarter-final against Ireland with eight caps compared with Ronan O'Gara's 115.
But injuries to Stephen Jones and James Hook gave the Scarlet his chance.
"That's why we have continued with him [Priestland]," said Gatland.
Wales' New Zealander coach overlooked Priestland in favour of Osprey Dan Biggar before last season.
But having bided his time as understudy at Parc y Scarlets to 102-cap veteran Jones, 34, Priestland's initial chance to shine came at his long-time mentor's expense.
Jones withdrew with a calf injury minutes before Wales faced England on 6 August in their opening World Cup warm-up game and Priestland, who had been named at full-back, took over at 10.
Priestland's assured performance in Wales' 23-19 defeat at Twickenham was followed by a win seven days later in which he also impressed.
Hook took over at fly-half when Wales ended their pre-tournament campaign with victory over Argentina.
Priestland then went on to start three of Wales' Pool D games - the 17-16 defeat by South Africa, 17-10 win over Samoa before coming on in the heavy win against Namibia and returing to the starting line-up in the Fiji rout that followed.
But assured performances apart, Gatland had spotted admirable qualities in Priestland on their summer training camps in Poland.
And despite Hook having recovered from a shoulder injury and Jones' calf problem clearing up, Priestland has held on to the much-coveted pivot role.
Gatland said: "I made a comment to the other coaches when we were in Poland that this kid's got something a little bit different about him," Gatland said.
"He's got a calmness, a coolness.
"What he didn't have was the experience and he has been lucky enough with Stephen's and James Hook's injuries to have that opportunity to bed himself in a little bit and that's why we have continued with him."
However, Priestland's recollections of the 6 August Twickenham encounter suggest that his "calmness" was severely tested.
"I was in a bit of a panic about remembering the calls," said Priestland.
"That day the players around me, especially Mike Phillips and Jamie Roberts, helped me.
"It was probably five or six minutes before kick-off [that I was told I would be playing fly-half]. 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.
"It is still a bit surreal to turn up here and see all these [media] microphones. 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 also hopes to win his personal duel with Ireland fly-half Ronan O'Gara, admitting the Munsterman has come out on top in their previous battles at Pro12 League level.
"Usually, I've been at the wrong end of the result when I have played against him. Hopefully, it will be different this weekend," said Priestland.
Wales: Leigh Halfpenny (Cardiff Blues); George North (Scarlets), Jonathan Davies (Scarlets), Jamie Roberts (Cardiff Blues), Shane Williams (Ospreys); Rhys Priestland (Scarlets), Mike Phillips (Bayonne); Gethin Jenkins (Cardiff Blues), Huw Bennett (Ospreys), Adam Jones (Ospreys), Luke Charteris (Dragons), Alun Wyn Jones (Ospreys), Dan Lydiate (Dragons) Sam Warburton (Cardiff Blues, capt), Toby Faletau (Dragons).
Replacements: Lloyd Burns (Dragons), Paul James (Ospreys), Bradley Davies (Cardiff Blues), Ryan Jones (Ospreys), Lloyd Williams (Cardiff Blues), James Hook (Perpignan), Scott Williams (Scarlets).
Ireland: R Kearney (Leinster); T Bowe (Ospreys), B O'Driscoll (Leinster, capt), G D'Arcy (Leinster), K Earls (Munster); R O'Gara (Munster), C Murray (Munster); C Healy (Leinster), R Best (Ulster) or S Cronin (Leinster), M Ross (Leinster), D O'Callaghan (Munster), P O'Connell (Munster), S Ferris (Ulster), S O'Brien (Leinster), J Heaslip (Leinster).
Replacements: S Cronin (Leinster) or D Varley (Munster), T Court (Ulster), D Ryan (Munster), D Leamy (Munster), E Reddan (Leinster), J Sexton (Leinster), A Trimble (Ulster).
Referee: Craig Joubert (South Africa).