South African newspaper Beeld reported Williams will fight heavyweight Francois Botha on November 17 in Durban.
Speculation has long surrounded the prospect of a fight against 43-year-old professional Botha, known as the "White Buffalo".
Boxing promoter Thinus Strydom told the newspaper final details around the fight had been completed this week.
Botha is a hardened boxer, having compiled a record of 48 wins from 58 fights, including 29 knockouts.
Opponents have included Lennox Lewis, Mike Tyson, Michael Moorer, Axel Schulz and Wladimir Klitschko.
Williams is comparatively new to boxing but holds the New Zealand Professional Boxing Association heavyweight title after beating American Clarence Tillman III in Hamilton in February.
After starring in the Chiefs' victorious Super Rugby campaign this year, Williams will farewell New Zealand with two All Blacks Tests, starting with Saturday night's Bledisloe Cup match in Sydney.
He leaves next month to begin a lucrative stint with the Panasonic Wild Knights club in Japan and will return to rugby league in the NRL next year.