Rugby mates Quade Cooper and Sonny Bill Williams have set up a mouth-watering clash© Brendon Thorne/Getty Images

Quade Cooper admits he faces a big challenge to establish himself in Australia's rugby sevens team for the Rio Olympics, as good mate Sonny Bill Williams has confirmed.

France-based Cooper returned to Sydney on Tuesday and immediately began training with the Australian squad for his debut in this weekend's Sevens World Series round at Allianz Stadium.

The former Wallaby playmaker has only secured a release from his club Toulon to play in three rounds of the World Series, giving him little time to adapt to the very different tactics and demands of the high-paced sevens game.

Landed and straight onto the paddock! Great to meet the lads and have a run around! pic.twitter.com/1QacCgiG0N

— Quade Cooper (@QuadeCooper) February 3, 2016

"It's a game I haven't played much of and I am under no illusions that it is going to be difficult," Cooper said.

Cooper revealed he had already spoken to Williams, the dual international superstar who made an up and down sevens debut for New Zealand in the World Series event in Wellington last weekend. Williams showed patches of brilliance but also made two big errors in the final before being replaced which almost cost his team the title.

"He [Williams] said it was tough and he said he is still a rookie," Cooper said. "I watched a few of his games and saw how well he went in patches - it's like anything, you learn as quick as you can and you adapt as fast as possible.

"There is still so much learning to do and I think for both myself and him it's very similar to boxing. When we started boxing we knew very little, and still do, but we put ourselves in that position to learn and that is where the challenge comes - in just being able to learn on the run."

Australian Sevens coach Andy Friend couldn't guarantee Cooper will get much playing time at this weekend's event, where the Australians will be desperate to impress before sell out home crowds. He also stressed Cooper would not be a walk-up start in the side in the lead up to Rio despite the great efforts the Australian Rugby Union has gone to secure his limited release from Toulon.

Unlike Cooper, Williams has committed fulltime to New Zealand's Sevens program, skipping Super Rugby this season, as he adapts to the game.

The inaugural Sydney 7s sold out nine days out from the tournament, with Australian Rugby Union chief executive Bill Pulver describing the response from fans as "extraordinary".