Prince William admitted he "got up like a puppy" when Taylor Swift asked him to sing with her.

It was 2013 when the British royal was led to the stage by the American singer during a Centrepoint charity event at Kensington Palace for a rendition of "Livin’ on a Prayer" alongside Jon Bon Jovi. At the time, even royal aides admitted William’s performance was completely "off the cuff."

Eight years later, the 39-year-old is opening up about what really happened that night.

"So, around about – it must be nearly 10 years now – I can’t believe I’m actually telling this story," said William on a special Christmas episode of Apple Fitness+’s "Time to Walk" series, which premiered on Monday.

PRINCE WILLIAM HAD A ‘CYBER RELATIONSHIP' WITH BRITNEY SPEARS, LAUREN BUSH BEFORE KATE MIDDLETON: BOOK

Prince William Taylor Swift puppy

Prince William, Duke of Cambridge, sat next to Taylor Swift during the Winter Whites Gala In Aid Of Centrepoint on November 26, 2013, in London, England. (Photo by Dave J Hogan/Centrepoint/Getty Images)

"I went to a charity fundraising gala for Centrepoint, which is a young homeless charity that I am very fond of and have supported for years," the father of three explained. "It’s an annual fundraiser, and I turn up, and Jon Bon Jovi and Taylor Swift are at the event, which nearly knocked me off my feet as well."

"When I sat down to watch Jon Bon Jovi do his performance, I thought, ‘That’s it. My job is done. I’ll get dinner in a minute, and I might be able to have a chat [with] some people, and you know, I’m off-duty a little bit now,’" William recalled. "Little did I think what was going to happen next."

"I’m sat next to Taylor Swift," he continued. "She’s on my left. And after Jon does his first song, there’s a pause, and she turns to me. She puts her hand on my arm, looks me in the eye, and says, ‘Come on, William. Let’s go and sing.’"

And William did exactly what the 31-year-old told him to do.

CLICK HERE TO SIGN UP FOR THE ENTERTAINMENT NEWSLETTER

Prince William Taylor Swift Jon Bon Jovi

Taylor Swift, Jon Bon Jovi and Prince William, Duke of Cambridge perform during the Winter Whites Gala In Aid Of Centrepoint on November 26, 2013, in London, England.  (Photo by Dave J Hogan/Centrepoint/Getty Images)

"To this day, I still do not know what came over me," William insisted. "Honestly, even now I'm cringing at what happened next, and I don't understand why I gave in. But, frankly, if Taylor Swift looks you in the eye, touches your arm, and says, 'Come with me…' I got up like a puppy and went, 'Yeah, OK, that seems like a great idea. I'll follow you.'"

"I walked up on stage in a trance, and then sort of halfway through Jon Bon Jovi's ‘Livin' On a Prayer’ song, I wake up," he shared. "And I'm thinking to myself, 'Am I standing on the stage singing 'Livin' On a Prayer' when I don't even know the words?'

"But the Centrepoint young guys and girls were there all loving it and cheering away. So I thought, 'Well, if they're enjoying it, then the night is for them. So sod it. I can't be the doofus who's going to ruin it for everyone.'

"And so I desperately try and remember some of the words and sing as hard as I can. Beneath my black tie, there was a lot of sweating going on. I felt like a swan, where I was trying to keep myself composed on the outside, but inside, the little legs are paddling fast."

CLICK HERE TO GET THE FOX NEWS APP

Prince William duet

Prince William admitted he wasn't prepared for the duet. (Photo by Dave J Hogan/Centrepoint/Getty Images)

William noted that despite being born in the public eye, he wasn't prepared to put his vocal skills on display in such a public platform.

"Now, a lot of people might think that I'm comfortable on stage," said William. "When I do speeches and things like that, I've done so many now, they're not a problem. But I've not done singing. At times, when you're taken out of your comfort zone, you've got to roll with it. And I think we've gotten to the stage in this life where we do micromanage ourselves. We do worry about: how do we look on social media? Who said what about me? What am I wearing? There's so many pressures, but I think making a fool of yourself is OK."

"It's OK to not take yourself too seriously and have those moments where you let go and you just go, 'Do you know what? I'm okay with this,'" William revealed. "So, yeah, go on and have a laugh. And don't go and watch the video of me singing. Or do go and watch it, but just laugh to yourself about how I was feeling."

Earlier this year, William teamed up with Apple Fitness+ to record one of their audio walking experiences. The series features personal stories from well-known personalities that are aimed to encourage people to walk more.

PRINCE WILLIAM AND KATE MIDDLETON WILL LIKELY VISIT THE U.S. IN 2022 TO BOOST THEIR POPULARITY, SOURCE CLAIMS

Prince William Taylor Swift

Prince William said he followed Taylor Swift "like a puppy" on stage. (Photo by David M.Benett/Centrepoint/Getty Images for Centrepoint)

William took listeners on a walk through Queen Elizabeth II’s Sandringham Estate in Norfolk, past St. Mary Magdalene Church and eventually in Anmer, where he and his wife Kate Middleton share a country home with their children.

William, who has used his platform to raise awareness on mental health, chose three charities to receive a five-figure donation from Apple: Shout in the UK, Crisis Text Line in the USA and Lifeline in Australia, People magazine reported. According to the outlet, Crisis Text Line and Shout provide free, 24/7 confidential support for people in crisis via text, and Lifeline provides free 24-hour confidential crisis support and suicide prevention services.