The 33-year-old will fight Australian television personality Stu Laundy in an exhibition match in Sydney on December 1 – a week after the All Blacks' final test of the year against Italy in Rome – a bout which is designed to raise money and awareness for the growing problem of homelessness here and across the ditch.
It is an event designed to leverage off Williams' profile as a high-profile athlete with the ability to continually divide opinion. He knows that and knows he will be judged again as a result but he's comfortable with it because the cause is close to his heart.
The announcement comes after the Blues confirmed Nonu would make his third return to the franchise next season to link with Williams and head coach Tana Umaga. Nonu, who played 103 tests, will turn 37 next year but still has it after his stint in France with Toulon according to his former teammate.
"I caught up with him in Wellington before it was announced to get a gauge on what his thoughts were and how he was going to come into the environment and he's positive, he wants to make a difference and he wants to succeed – that's all we want to do," Williams told the New Zealand Herald.
"Ma'a has really looked after his body over the years – especially in his latter years. I remember with the All Blacks him being really professional and diligent with his nutrition and what-not. I've got no doubt that he'll be able to compete. It's just about getting out there and getting that combo back again.
"We don't have the luxury of having 10 All Blacks in our team – that's the reality. The more experience we can gain the better. We've got talent, yes by all means. We've got work ethic, we want to work hard as a club, we want to go in the right direction and getting players like him with his experience I believe is a real positive."
Wrist, knee and shoulder injuries have curtailed Williams to only seven games this year – one test against France in Dunedin and six matches for the Blues – so he was extremely disappointed to be given the news last Thursday morning by head coach Steve Hansen that he wouldn't be playing the Springboks on Wellington.
"Going in on Monday night I was really excited about the prospect of being chosen," he said. "On Tuesday when I woke up, wow … I had the shakes and all that stuff and I went to the doc's room and he said 'you've got tonsillitis mate, you'll be out for the count for a couple of days. I don't think you'll be right for the weekend. You have to be right to train on Tuesday'.
"I delved into recovery mode. By Thursday I was okay and Doc said '60 or 70% today and you'll be right on Saturday'. I was pumped but the coach came and snipered me and said 'mate, we can't pick you, you've been out for two or three months, how are we going to pick you to play? It could be 75 minutes off two or three days' training'. It was tough, the younger me probably would have got all salty."
Williams, the father of three children, is determined to leave a legacy that is more significant than being a former All Black or professional boxer, and that makes him unique in this part of the world.
"As New Zealanders we tend to shy away from the spotlight because people will look and judge. The more you can get comfortable in your own skin, the more you'll step out for causes like this that are close to your heart.
"I've always felt like I've been a good person – just a little bit misunderstood. Now I don't care, bro. I'm doing something good … people will point the finger and say 'look at this dickhead, who does he think he is up on stage with a beanie on his head…'
"I'm really excited about working some more with Chris and the City Mission because I want to show my kids that way of living – give them gratitude and empathy."