US boxing superstar Floyd “Money” Mayweather fights undefeated Japanese kickboxing prodigy Tenshin Nasukawa on Monday, a New Year's Eve multi-million-dollar rumble outside Tokyo to be held under boxing rules.

At 41, Mayweather is more than twice the age of his Japanese opponent, the 20-year-old known as “Ninja Boy” but will carry a more than 4kg (9lb) weight advantage into the ring.

The bout in Saitama, north of Tokyo, will be fought over three rounds of three minutes but there are no judges and no winner will be declared unless there is a knockout.

The bout will also not count on the record of either fighter — allowing both men to retain their cherished unbeaten records -- and is being promoted as a pure exhibition match.

It will also be boxing only — putting Nasukawa at a disadvantage, with the kickboxer reportedly liable to a $5 million fine if he aims a kick at Mayweather.

The purse has not been officially disclosed but media reports have claimed that Mayweather is making $88 million for the nine minutes of action.

At a pre-fight news conference, a relaxed-looking Mayweather said he could complete three rounds “in his sleep”.

“It is all about entertainment. I don't worry about anything,” said Mayweather, a legend in the boxing world who has a perfect 50 in and zero defeats record with 27 knockouts.

The relaxed American said he would not be concerned even if he were to be floored by his Japanese opponent.

“Me getting knocked out or me getting knocked down ... I don't worry about it at all. If that does happen, I mean that's entertainment. That's all we need to see,” said Mayweather.

'Gym rat'

Virtually unknown outside the kickboxing ring in his own country, the bleach-blond Nasukawa from Chiba near Tokyo also boasts an unbeaten record.

He won the world junior karate championship, made his professional kickboxing debut at the age of 16 and has a 28-0 record with 21 wins by knockout.

Promoters RIZIN have hailed him as “perhaps the best combat sports prospect the country has ever seen” and the 20-year-old himself has claimed he has a “punch that boxers don't have.”

“I want to be the man who changes history. I'll do that with these fists, with one punch -- just watch,” Nasukawa said.

Even Mayweather appears impressed, describing his opponent as “an unbelievable talent” and praising his lithe, “gym rat” physique.

“He is very young. He is very active... very tough. I had a chance to see some highlights (of his performance) and I was impressed with Tenshin. I was very impressed with him.”

Nasukawa has dismissed the idea that the boxing rules will count against him, saying: “All I have to do is to give 100 percent ... Since Mayweather is expected to dodge my offence, I really want to hit him.”

This is the second time Mayweather has been coaxed out of retirement. Last year, he knocked out mixed martial arts fighter Conor McGregor in the 10th round of a super-hyped boxing match.

There are also unconfirmed rumours Mayweather could take on reigning UFC lightweight champion Khabib Nurmagomedov, who said last month any match-up would be “twice as interesting” as the McGregor fight.