Floyd Mayweather has revealed that he is aligning with the Rizin Fighting Federation, a Japanese mixed martial arts company, and will fight kickboxing star Tenshin Nasukawa at the organization’s year-end show on New Year’s Eve in Japan.

The shock announcement came at a Monday news conference in the downtown Tokyo district of Roppongi, where Rizin officials introduced both Mayweather and Nasukawa, the undefeated 20-year-old kickboxing prodigy who joined the MMA promotion in 2016.

“It wasn’t easy to make this happen,” a subdued, polite Mayweather said. “But we told the people anything is possible, so now we’re here and we want to make sure that we give the people in Tokyo what they want to see: blood, sweat and tears.”

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Promoters said the fight between Mayweather and Nasukawa will headline the Rizin 14 card to be staged on 31 December at the Saitama Super Arena.

Mayweather, who turns 42 in February, captured world championships in five divisions, but has not fought since a 10th-round knockout of UFC star Conor McGregor under boxing rules in August 2017. He announced his retirement after the McGregor fight with a 50-0 record as a professional, but has since teased comeback fights against old nemesis Manny Pacquiao and UFC lightweight champion Khabib Nurmagomedov.

Nasukawa, a southpaw who is unbeaten in 27 kickboxing fights and four MMA bouts, has won titles across several promotions in the flyweight (125lb) and bantamweight (135lb) divisions, which raises an obvious question about the contracted weight for December’s fight: Mayweather hasn’t fought below the 147lb limit since his 2005 meeting with Arturo Gatti.

Also unclear following Monday’s announcement was if the fight will take place under the rules of boxing, kickboxing, mixed martial arts or some blend of the three.

“We’ll talk about that, we’ll get that situated within the next couple weeks,” Mayweather said when asked about the weight and the rules. “As far as the weight class, we’re not really worried about that. When it’s all said and done, it’s all about me going out there and displaying my skills against another skillful fighter.”

He added: “Throughout my career I fought at a heavier weight class but I always was really a smaller fighter. It’s really all about the skills, it’s really not about the weight.”

Rizin chairman Nobuyuki Sakakibara echoed Mayweather’s confidence in completing the negotiations: “As far as the rule set and the weight and the rules, we still have some work cut out for us. But it will be a great fight that everybody will be so excited to see.”

Nasukawa, for his part, appeared to relish the chance to fight with Mayweather, admitting his surprise when he was offered the opportunity and saying he “would not mind” if it were decided that kicks were outlawed.

“I think my role as a fighter is to make sure that fighting here in Japan gathers a lot of excitement,” he said. “When I was offered this card, I accepted it right away without any delay whatsoever.”

He added: “You might be surprised after hearing this announcement, and honestly speaking I’m a little surprised myself. Nobody has defeated my opponent in the past and I would like to be the man who makes history. My punch can change history and I will show that to you.”

Mayweather, who won his first world title when Nasukawa was less than two months old, was conciliatory in his assessment of the young champion, saying: “This kid right here is very, very special. He’s powerful, he’s fast, he’s strong. I’m older now. I think that when it comes to experience, I have more experience on my side. But when it comes to youth, youth is on his side.”

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No financial details were disclosed during Monday’s news conference, a conspicuous omission given Mayweather’s proclivity for bragging on his paychecks when promoting a fight. One Entertainment CEO Brent Johnson, the entertainment executive who helped broker the deal for Mayweather and who sat beside the boxer on Monday, suggested that “long-term partnerships and opportunities” between Rizin and Mayweather’s nascent promotional company were the priority over a one-time cash grab, but admitted the fight would “have to make money to make sense”.

Sakakibara, who established Rizin in 2015, rose to prominence as the co-founder and president of the Pride Fighting Championships, the wildly popular and influential Japanese mixed martial arts organization that was sold to the parent company of the Ultimate Fighting Championship (UFC) in 2007. While the companies were initially intended to co-exist as independent entities with occasional crossover events, the UFC instead absorbed many of Pride’s top fighters before abruptly folding the operation.

The bout will mark Mayweather’s first fight on foreign soil since his amateur days and the first time he’s fought outside his adopted home of Las Vegas since his 2005 match with Sharmba Mitchell at the Rose Garden in Portland, Oregon.