Sonny Bill advises Quade Cooper to 'buy a new set of lungs'

Sonny Bill Williams has told Quade Cooper what's most needed to surprise the doubters in his sevens gamble: "Buy a new set of lungs".

Cooper has been a frustrated yet captivated spectator at the Sydney 7s after being controversially overlooked by coach Andy Friend on physical grounds, but he did get some valuable insight into the systems and structures of the Australian team as well as a first-hand look at the gut-busting fitness requirements in the high-energy game.

Close mate Williams, cutting his teeth in his second tournament for New Zealand, also underlined the biggest challenge to adapt from traditional 15s play.

"He just said 'if you get a chance buy a new set of lungs'," Cooper told Fox Sports.

"Sevens is very hard on the body so for me I'm just going to do as much fitness as I can and just prepare myself as well as possible."

Largely prevented from making his sevens debut due to his delayed release by French club Toulon, Cooper is talking as though he'll be cleared to play in Las Vegas and then Paris and London in May.

New Zealand coaching guru Sir Gordon Tietjens is among those who doubts that's enough exposure to adapt to the game, and he has warned Australia could be badly exposed by the gamble.

But South Africa's big-name convert, Francois Hougaard, believes the mercurial playmaker is ideally suited to sevens.

"Quade Cooper is a phenomenal athlete and he is the type of guy who adapts very quickly to any system," Hougaard said.

Unlike Cooper, Hougaard and Williams are sacrificing their Super Rugby seasons to make the Rio Games.

"I'm just grateful to be in such a great side," Williams said. "I'm still learning the ropes but I'm loving the game of sevens at the moment.

"I knew I had to lose a few kilos and that after a few tournaments my fitness would get better."