The Chiefs utility back is the first, and so far only player in world rugby to take advantage of the eligibility loophole created when Sevens became an Olympic sport.
Because the Olympic eligibility laws are different to World Rugby's, a ruling was made that players whose eligibility had been captured could play four Olympic-qualifying Sevens tournaments and switch allegiance at all levels of the game.
Nanai-Williams, having played Sevens for New Zealand, decided late last year to make the switch to Samoa - the country in which both his parents were born.
"It took a bit of time to be honest," he says of his decision to throw his lot in with Samoa.
"Once I heard about it I thought about it for a while. I didn't really jump at it for maybe a month or so then I thought I might as well give it a go.
"I had to ask a lot of people - a lot of rugby people who had played for Samoa - and people that I look up to.
"Guys like Tana [Umaga] and ask their advice because it has never been done before and I am the only one and still the only one that's done the switch."
Umaga's advice was for Nanai-Williams to follow his heart. He listened to his head, too, though before committing. Nanai-Williams grew up in New Zealand and his dream had always been to be an All Black.
But after four seasons with the Chiefs where his elusive running and electric pace had been given ample opportunity to impress, he didn't have any sense a call up to the national team was imminent.
For all that he had given - for all that he could exploit the tiniest holes in the meanest defences from centre, wing or fullback - Nanai-Williams could see the path ahead to national honours in New Zealand was blocked.
After a month of deliberation, both his heart and head were telling him Samoa was his future.
"Once I made the decision, I have been loving every minute of it," he says.
His journey reached a destination of sorts today when he was named to win his first cap against the All Blacks. The hard work suddenly all became worth it.
The decision, not that he ever doubted it, made perfect sense.
"It is a bit of an arrival," he says. "When you play rugby, you always want to win. I'm going to enjoy every minute and soak it up. The All Blacks are the best team in the world."
They also include his cousin, Sonny Bill Williams, and a host of other players Nanai-Williams knows well.
But he doesn't see that there will be much to gain from his knowledge and his only prediction about Wednesday's test is that it will be draining given the likely ferocity and pace.
"I have had a few texts already from Sonny and Liam Messam. I have thrown some back as well. But we are focusing on our game and how we are going to play.
"We have to make sure we match them in every aspect. We believe in our game and that we can put in a good performance. I'd say it will be a fast game. It is going to be a big day, a big occasion."
- By Gregor Paul of the Herald on Sunday in Apia
