The rugby league convert, who spent time with Toulon before returning to New Zealand, has become one of the biggest names in the sport.
Amazingly, as well as his rugby career with the Crusaders and the All Blacks, Williams is a professional boxer.
The 25-year-old has won all three of his previous professional bouts and will fight again during a bye week for the Crusaders.
Williams will take to the ring at Waitakere's Trusts Stadium on June 6 against a yet unknown opponent.
"I'm not a world beater at this stage." said Williams.
"I will definitely be in the ring with someone who could knock me out."
The fight has been build as the 'Clash for Canterbury', with Williams donating NZ$100,000 of his purse to the Christchurch Earthquake appeal.
"I'm blessed to be able to make a good earning as a sportsman," added Williams.
"I want to help the hardships people have gone through. This is a no-brainer for me. I feel morally obliged. I didn't come from much and I feel blessed to be where I am now."