Sonny Bill gunning for Olympic gold

He also made himself unavailable for Super Rugby and the June Test series against Wales this year to focus on the sevens.

To make things better for the Williams family, Sonny Bill will be accompanied to Rio by his sister Niall, 28, who was a New Zealand touch rugby player for several years before switching to sevens last year.

"I'm proud of her, but at the same time there are a lot of media here and a lot of spotlight on myself and my sister, but for us it's not about us two," said Sonny Bill.

"To be able to say you're an Olympian and get a medal, that'd just be another level," Williams told ESPN. "It would be unbelievable. I get butterflies just thinking about it."

Coach Gordon Tietjens, who has guided New Zealand to 10 World Sevens titles, rated his side as medal contenders despite finishing third in this year's world series.

"It's going to be a challenging environment for all of us and all the teams as it's such a new experience," he said, with the New Zealand men's team drawn in Pool C with Britain, Kenya and Japan.

"Sevens is very competitive, so it's important that we perform and if we perform to our potential, we'll have a really good chance of succeeding in Rio."

Sonny Bill personifies the type of oval ball player any sevens coach would warmly welcome with open hands: a towering athlete offering physicality, dynamism, speed and offloading ability, technically strong with a capacity to read a game.

It is no surprise that a raft of touted 15-a-side players missed out on selection, having failed to gel quickly enough with the sevens format, played on a full-size pitch and not for the faint-hearted, the slightest defensive error likely to be fully exploited.

South Africa overlooked veteran winger Bryan Habana and Fiji decided not to go with former Australian rugby league and American football star Jarryd Hayne.

Hayne, who in May ditched a career in American gridiron with the San Francisco 49ers to join the Fijians, said he had not given himself enough time to transition into sevens, evidence of the gulf in athleticism required for the high-octane game.

The NFL (National Football League) will be represented, however, in the shape of New England Patriots defensive back Nate Ebner, who also worked on special teams as his outfit captured last year's Super Bowl crown.

Ebner was the youngest-ever US Sevens player at age 17 before going to Ohio State, where he split time with rugby and American football until he started his NFL career, never losing his love for rugby.

It was indeed the Americans who won gold when rugby was last played in the Olympics back in 1924 in the 15-a-side format which debuted in 1900.

Also in their squad is Carlin Isles, dubbed "the fastest man in rugby" having clocked a personal best of 10.12 seconds for the 100m during his track career, a time that would have got him into the semi-finals of the 2012 London Games.

His switch to rugby sevens was a tough learning curve, but he adapted quickly and his legend took off in late 2012 when a youtube video showcasing Isles' speed went viral. To date, the video has garnered nearly seven million views.

For the competition itself, top seeds are two-time defending world champions Fiji, chasing a first-ever Olympic medal.

The islanders were handed what should be a relatively straightforward route into the quarter-finals in Pool A, where their opponents will be Isles' United States, Argentina and hosts Brazil.

Pool B will see southern hemisphere rivals South Africa and Australia vying for supremacy alongside France and surprise qualifiers Spain.

In the women's tournament, top seeds Australia will play the US, Fiji and Colombia in Pool A while New Zealand, France, Spain and Kenya contest Pool B.

Canada, Great Britain, Brazil and Japan complete the field in Pool C.

The top two teams from each pool qualify for the knockout rounds, along with the two best third-placed sides at the 15,000-capacity Deodoro stadium.