Sonny Bill Williams has had his first taste of Sir Gordon Tietjen's famed gruelling fitness regime - and is already feeling the pain.

The much decorated cross-code superstar is aiming to add an Olympic gold medal to his already bulging trophy cabinet by concentrating on sevens rugby in 2016 and is looking to make his New Zealand debut at the Wellington Sevens on the World Series circuit later this month.

That challenge is requiring a whole new demanding fitness regime overseen by Tietjens who is legendary for his gut-busting training sessions and insistence his teams are always the fittest on the park.

And even the superbly conditioned Williams, a committed trainer whose professional approach inspires team-mates, is feeling the pinch judging by his latest social media posting.

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The twice World Cup-winning All Black tweeted a picture of himself recovering with baby daughter Imaan by playing on the floor after a sevens training session.

"Sevens rugby training ain't no joke! Even my daughter needs to rock the recovery socks haha" he tweeted while wearing recovery socks.

New Zealand rugby officials are hoping Williams' presence in the Wellington tournament will inspire ticket sales which have been poor to date. They will also be hoping his ball-running and playing skill rubs off on the national sevens squad which has made its worse ever start to a World Series in a year where Olympic gold is up for grabs in Brazil in August.