He's one of the fastest sprinters in Australian history and is confident he could have cracked the magical 10-second barrier for the 100 metre in coming years.

But now Trae Williams - the speedster known as “Quadzilla” for his enormous thighs - is aiming to attend the 2020 Olympics as a rugby player, after signing on with the Australian men’s sevens team for two years.

Williams, 22, made the shock call to switch running spikes for footy studs after being recruited by Aussie men’s sevens coach Tim Walsh and assistant Steve Hoiles, who decided to go majorly left field in their hunt for genuine speed in their squad.

Williams is no stranger to footy, having played rugby and rugby league as a schoolboy and trialled with the Under 20s programs of the Cowboys and the Broncos in 2014 and 2015.

After coming third in the 100m at the Youth Olympics in 2014, Williams chose to pursue athletics and last year clocked a sizzling 10.10 seconds at the Commonwealth Games national trials.

That time put him fourth on the Australian all-time list behind Patrick Johnson and Matt Shirvington, and is even quicker than the personal best of noted USA sprinter-turned-sevens speedster Carlin Isles (10.13).

It’s understood Hoiles came across Williams by researching Australia’s fastest sprinters and after learning about footy-playing past of "Quadzilla", the former Wallaby made contact and then flew to Brisbane for a meeting.

Williams then came to Sydney for a day’s testing and was subsequently signed up.

Despite being regarded as likely pick for the Australian sprint team for the Tokyo Olympics, Williams told RUGBY.com.au the chance to play rugby for his country was "too good to pass up”.

"It was definitely a big decision. I obviously grew up playing league and then through high school played rugby so I have always had that passion for it. And it’s an Olympic sport as well,” Williams said.

"So to have that opportunity to go to the Olympics and play rugby is a once-in-a-lifetime opportunity, so I had to take it.

"I had a very good chance of making it to the Olympics in the 100m and the 4x100m, but it came down to what I wanted to do, and I have a passion for rugby.

“Steve (Hoiles) just rang me up and offered me the chance to come play for the sevens team. I obviously had a chat with my family and my friends and they said “do what your heart tells you”. So I took the opportunity and I can’t wait.

"The guys are a good bunch of guys and a good team, so hopefully I can be another extra piece of the puzzle to help achieve some great things.”

Williams has been contracted to the Australian sevens full-time squad and will fly to London this weekend to spend the tournament in camp with his new teammates.

“We’ve identified that a key area we need to improve on is adding more speed to our squad and Trae is the combination of speed and physical ability,” Walsh said in a statement.

“Players with sprinting backgrounds have made excellent converts to the game such as the USA’s Perry Baker and Carlin Isles as well as our very own Ellia Green and we believe Trae’s transfer will be just as successful.”

After testing the waters of playing rugby league after school, Williams chose sprinting but the thought of playing football never left his mind.

"Me and my training partners have been talking about it for ages, having the opportunity to be able to do that,” Williams said.

"When Steve rang me up after nationals this year and I thought “crap, maybe this is actually something I will be able to do."

“In '14 and '15, I was just making my first Australian teams in athletics and it was a harder choice to choose between the sports.

"But it wasn’t until recently, or basically when Steve rang me, that I had a really good think about it. I feel like that I have done the most that I can in athletics, it was time to try and different sport.

"I still feel like I am at the peak of my (sprinting) career but when the opportunity came along to play for Australia, it was hard to say no. That’s just the main thing that made me decide to play rugby.”

Williams, who can hack squat 525kg, was on the verge of a major breakthrough last year but injury hampered his preparation for nationals this year.

He ran in the recent IAAF relay titles and maintains his best was still to come over the 100 metre sprint, namely that he could have been the second Australian behind Johnson to run under 10 seconds.

"I have full faith that I could have broken the ten second barrier,” Williams said.

"I had a bit of injury this year heading into nationals and that affected me, but if I had continued to train until the next Olympics I could have gone sub-10. But that’s fine. I am just focussing on rugby now and running for the team.”

Williams hasn’t played any serious footy since school but said he is confident he has the skill base and defensive mindset to get quickly up to speed.

Williams knows all about Isles, who was also a track sprinter in the USA before switching to rugby.

He’s watched all Isles’ highlights on YouTube and yes, he can’t wait to go toe-to-toe with the US Eagles speedster.

"I have definitely looked at all the highlights. His footwork and his acceleration are impressive, so to get to something like that level would be great,” he said.

"It’ll be a good match up with Carlin and (Perry) Baker and I’d be keen to give it a go.”

There won’t be any juggling of sprinting and playing rugby for Williams, other than keeping up the athletics training to maintain his speed.

"I spoke to Tim Walsh and they obviously want me for my speed, and I will keep doing running training to keep that up. But obviously then also all the rugby training too,” Williams said.

"I don’t think you can do both at the same time. I will obviously keep my training up to maintain the pace, but I will be focussing on rugby from now on.”