On 13 November 2012, Williams confirmed he would be returning to rugby league after signing a one-year deal with the Sydney Roosters for the 2013 NRL season. His return was undertaken to honour a handshake agreement with Roosters chairman Nick Politis. It took over a month before his contract was officially approved and registered by the ARLC; he was "frogmarched into League Central to be grilled as part of the probe" into his contract. Williams's return coincided with the banning of the shoulder charge, a manoeuvre of which he has been described as the best and most famous exponent.
On 9 July 2012, Williams announced he would be playing for the Panasonic Wild Knights in the Japanese Top League during the 2012–13 season—with the allowance to have one boxing fight during the season—before returning to rugby league. The Panasonic deal was thought to be the largest one-season contract in rugby union history. He was also expected to be heavily involved in the promotion of the 2019 Rugby World Cup in Japan, the country being the main host of the event. On 9 September, a week after sitting out the Top League season opener to be given time to adjust after coming off recent All Blacks duties, Williams made his debut for Panasonic against NTT Communications Shining Arcs. On 22 September he scored his first try for Panasonic, while playing against Toshiba Brave Lupus. On 27 October, Williams scored his second try during Panasonic's eighth round victory over the Kintetsu Liners; however, the match would be his final game of the season after sustaining an injury when he landed awkwardly on his shoulder following a tackle.
On 8 February 2012, Williams was supposed to fight Richard Tutaki for the vacant New Zealand Professional Boxing Association (NZPBA) Heavyweight Championship title at Claudelands Arena in Hamilton, after Shane Cameron vacated it to move down to the cruiserweight division. However, it was later revealed that Tutaki was facing serious criminal charges; so he was subsequently dropped from the fight card. Williams's replacement opponent was then announced to be Auckland-based American Clarence Tillman III. The fight was dubbed the Battle for the Belt. Williams went on to claim the title belt by technical knockout after a left hook and a series of further blows on Tillman forced referee Lance Revill to stop the fight in the first round. Following the bout, Williams rejected an offer to join the boxing stable of fight promoter Don King.
On 24 November 2012, during the Japanese Top League's November break, Williams was due to fight South African former heavyweight contender Francois Botha at the Brisbane Entertainment Centre. However, the bout was postponed to 8 February 2013 after Williams required surgery for a right pectoral muscle injury sustained while playing for Panasonic on 27 October. The fight was for the vacant WBA International Heavyweight title. The event also marked the boxing debut of Williams's close friend and fellow rugby player, Quade Cooper, on the undercard. After dominating most rounds but on the verge of being knocked out in the last round, Williams went on to win the bout by unanimous points decision, with the judges scoring 98–94, 97–91 and 97–91 in his favour. However, the victory was marred by controversy because at late notice (and unbeknownst to most people) the fight was shortened to ten rounds, instead of the scheduled twelve. Australian National Boxing Federation committee member John Hogg later stated that the decision to cut the bout short was made just before the fight started, with the approval of officials and both Williams's and Botha's camps; however, Botha was not informed of the change by his own camp.