Floyd Mayweather Jr.’s Next Opponent? A Kickboxer

Floyd Mayweather Jr.’s retirement keeps growing weirder.

After announcing his retirement in 2015, he became a boxing promoter. Then he came back to the ring to fight Conor McGregor, a mixed martial arts champion. Now Mayweather, one of the greatest boxers in history, will apparently fight again.

On New Year’s Eve, Mayweather will go toe to toe — maybe literally — against Tenshin Nasukawa, an undefeated Japanese kickboxer, in Saitama, Japan.

Mayweather is now 41; Nasukawa is just 20. Despite his youth, Nasukawa has an impressive fighting résumé, with 27 kickboxing victories, dating to when he was 15, and a 4-0 record in M.M.A. Combat Press rates him as the eighth-best bantamweight kickboxer in the world.

Mayweather’s boxing match against the M.M.A. superstar McGregor was one of the events of the year in 2017, drawing attention that reached well beyond sports. Mayweather dominated McGregor, who had never boxed professionally, winning in a 10th-round knockout.

Fight fans will no doubt be looking forward to reading details of Mayweather’s next adventure. But despite the announcement on Monday, those details have yet to come.

Will the men face off in boxing, kickboxing or M.M.A.? (Mayweather fought McGregor under very slightly modified boxing rules.) At what weight will the men fight? (Mayweather fought in recent years at 147, and faced McGregor weighing 149 and a half pounds; Nasukawa weighs more like 125.) Will it be a megabucks fight? (Mayweather earned upward of $300 million for the McGregor fight.)

“As far as the weight class and the rules, we’ll talk about that,” Mayweather said at the news conference announcing the fight.

“We still have some work to do,” said Nobuyuki Sakakibara, the chairman of Rizin, the Japanese M.M.A. organization that is promoting the bout, in what seemed like a big understatement.

For a “retired” fighter, Mayweather has been busy. There has also been talk of a bout with Khabib Nurmagomedov, who beat McGregor in M.M.A. last month. The chance of that bout taking place seemed to fade over the last week when the Ultimate Fighting Championship president, Dana White, said it would have to be under M.M.A. rules and Mayweather dismissed that possibility.

There has also been talk of a rematch of Mayweather’s 2015 bout with Manny Pacquiao, which Mayweather won by decision. Pacquiao is now 39, but still active (not to mention serving as a senator in the Philippines) and a titleholder. He is scheduled to fight in January in Brooklyn.

The imprecise details of the Nasukawa bout calls to mind another time when a superstar American boxer went to Japan to fight under vague rules.

In 1976, the world champion Muhammad Ali took on a professional wrestler, Antonio Inoki, at the Budokan arena in Tokyo. Entertainment did not ensue.

When the bell rang, Inoki fell to the mat in a crablike posture and began kicking out at Ali’s legs. Ali danced around the ring looking for a chance to land a punch but not finding one. The bout continued for 15 interminable rounds and was scored a draw.