Williams will leave the Chiefs at the end of the Super Rugby season to join the Wild Knights on a short-term deal.
The 26-year-old cross-code convert will then play in the NRL in 2013 - with Williams having a "handshake agreement" with an unnamed Australian club, with the Sydney Roosters and Canterbury Bulldogs both linked to moves.
Williams, who will also continue his boxing career, said he initially turned down the Japanese attempt to lure him but they came back with an offer he could not refuse.
The dual-code international, a Rugby World Cup winner last October, accepted the move would likely end his All Blacks career but remained hopeful that would not be the case.
"The decision to come to the Chiefs was a pretty big thing, but it has really helped me, especially off the field," he said.
"I didn't think I would love rugby as much as I have.
"If things work out I would love to come back and play for the All Blacks in the future. But I know there is a group of players who are coming through.
"I've chosen my path now I've got to walk it."
All Blacks coach Steve Hansen has refused to rule out a return to the international set-up for Williams.
"He came to New Zealand rugby two years ago as a convert player and was similar to Brad Thorn in that regard," Hansen said.
"Sonny had high level skills and freakish abilities without a fully developed game.
"This season we have seen the birth of Sonny as a rugby player with his game understanding complementing his array of skills.
"His performance in the Test series against Ireland showed that he was world-class."
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The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
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