Kyle, 15, wowed judges on Saturday's show and went straight through to the semi-final after Walliams gave him the golden buzzer on the hit ITV show and admitted he'd made a mistake turning him down several years ago.
Now early footage of Kyle delivering a flawless performance of Bruno Mars' When I Was Your Man has emerged on YouTube - and has gone viral in the wake of the Sheffield singer's success on the show.
In the clip filmed when he was just 12, Kyle, wearing a Call Of Duty t-shirt and standing in front of a curtain, delivers a heart-rending rendition of the US chart star's hit.
The clip has already been watched more than 80,000 times.
It was revealed on Saturday's episode that Walliams had told the youngster to improve his singing voice three years ago when he was 12, the time the video was filmed.
But the comedian was made to eat his words as Kyle's flawless rendition of the emotional track Hallelujah wowed the judges.
Walliams then became the first judge to press the golden buzzer, which automatically sent Kyle straight through to the semi-finals.
After the 15-year-old had finished the song, Walliams said: “I thought it was really good. Well done Kyle.
"It was a really, really moving performance and I’m really glad that you came back and proved me wrong."
Simon Cowell added: “You gave every single ounce you could possibly give to that performance. This just shows you that when somebody grinds you down and says you are not good enough, you come back, and you look them in the eye, and you go 'I am good enough'."
Fellow judge Alesha Dixon described his story as being like a “fairytale.”
After the triumphant audition, Kyle told how his dream was made possible after his family paid for his tuition.
He said: “Back then I thought I was really good, the best thing since sliced bread.
“But David kind of put me in my place. I was absolutely gutted but he was spot-on what he said. I haven’t looked back since then to be honest.
“So I sat down and thought about it, and the day after I went to my mum and dad and I went: ‘I need singing lessons.’
“My nanan and grandad have paid for them every day since three years ago. It’s Â£15 every 30 minutes, and once a week, so it’s a lot of money.
“But they are thrilled for me now.
“They couldn’t be there as my grandad is in hospital with cancer, but he was waiting by the phone to hear, and he was so happy when I told him.
“I am going to dedicate my semi-final song to him, because he’ll be watching, and I 100 per cent want to try to win this for him.”