Having blown AIS coaches away with his raw power as an unheralded teenager, Roberts has slashed almost three seconds off his 100m time since coming to Canberra to train two years ago.

Magnussen is now the only man in the world who has gone faster in a textile suit, setting up the thrilling prospect of an Australian quinella at the London Games.

The pair first competed against each other as 16-year-olds and their similarities are uncanny – from their first names to their birth date, April 11, 1991.

While it promises to develop into one of the great rivalries of Australian swimming, Roberts said he was excited at the prospect of teaming up with Magnussen for a gold medal challenge in the 4x100m relay.

"Ultimately we are going to be competitiors in the 100 freestyle but the first event [in London for us] is the 4x100m relay so we're teammates first and foremost," Roberts said.

"I'm just happy the fastest man in the world at the moment and the one guy ahead of me is an Aussie, I've got someone to chase and I think we'll continue to push each other throughout our careers."

Magnussen shot to prominence by winning last year's world championships in Shanghai in a time of 47.63 seconds, while Roberts has continued to sneak under the radar until this year's Olympic trials when he finished second to the Missile – amazingly with a personal best 47.63.

It was 2.64 faster than the personal best Roberts had swum just two years earlier, at the Commonwealth Games trials.

AIS coach John Fowlie admitted Roberts had "exceeded expectations", but he refused to put a limit on the 21-year-old's potential.

"This won't be the last Olympics for James Roberts, this is just the beginning," Fowlie said.

"While a great result in London would be phenomenal, this guy could be a great swimmer for Australia for a long time to come."

Fowlie recalled the first time he saw Roberts, as an 18-year-old invited to Canberra to do testing with the Australian squad.

The boy from Tweed Heads had only been swimming competitively for two years, but blasted everyone out of the water – including former world record holder Eamon Sullivan – with his power testing.

Inside a week Roberts was offered an AIS scholarship.

"I'd never heard of him or seen him before and he posted the highest peak power output out of anybody in the testing we did. You could see there was something there with this guy," Fowlie said.

"Before he came to the AIS you'd almost say he'd done no real training so he was really an untapped resource.

"So when a guy that's six-foot-four and 90 kilos that doesn't train at all walks through the door going 50.2 [seconds] for 100 freestyle, you know you've got something good on your hands."

Troubled by "niggling shoulders" and sickness as a teenager, Roberts had been focusing on the 50m until he joined the AIS.

That's when coaches urged him to step up to "the big boys' event".

"I always struggled with the 100 actually and dreaded racing it, the 50 was all I wanted to do," Roberts said.

"But since I moved to the AIS they got rid of that mentality and drilled it into me that the 100 is where you want to the be, that's where the relay positions are. Moving to the AIS has really forced me to become consistent with my training.

''I was swimming less back home and thinking I could get away with it and was on that cusp of breaking through.

"Anything's possible in the sport . . . you never know when you're going to stop improving.

"It's exciting to see where I've come already.''