Undefeated American boxer Floyd Mayweather returns to the ring tonight in an exhibition bout against young Japanese kickboxer Tenshin Nasukawa.

It will be the first time Mayweather has fought since stopping Conor McGregor in 2017. Here's what you need to know about the match up.

The bout

The pair will fight over just three three-minute rounds in an exhibition. There will be no judges, so if neither fighter is knocked out, there will be no winner named.

If one of the fighters is knocked out, the loss will not count toward their professional record.

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At 41 years of age, Mayweather is more than twice the age of the undefeated Japanese fighter. The 20-year-old Nasukawa, nicknamed 'Ninja Boy', has won all 28 of his professional kickboxing bouts - 22 by way of knockout - and is unbeaten in four MMA fights as well.

The fight will be an out and out boxing match. No kicking will be allowed and Nasukawa will reportedly facea $7m fine if he aims a kick at Mayweather.

The bout headlines a 14-fight card, which gets underway at 7pm NZ time.

How can I watch it?

The card can be purchased and streamed through FITETV.

If there's no winner, what are they fighting for?

The purse for the fight is a large one. While an exact amount has not been disclosed, media reports have suggested that Mayweather is making $125 million for the nine minutes of action - that's more than $230,000 per second.

Mayweather: 'I can do three rounds in my sleep'
- AP

Floyd Mayweather is promising entertainment when he fights Japanese kickboxer Tenshin Nasukawa in three rounds of exhibition boxing tonight but not necessarily a knockout.

"If it happens, it happens," Mayweather said on Saturday when asked whether he was going to knock out Nasukawa.

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But the American was adamant that his opponent wasn't going to knock him out or down.

"Throughout my years at boxing, I never touched the canvas once," Mayweather, 41, told reporters at a Tokyo hotel, brushing off the upcoming event as "just another day."

"I can do three rounds in my sleep," he added.

Neither kicking nor official results are part of the Rizin Fighting Federation match, but a knockout is possible under the special rules.

Mayweather had initially appeared to walk away from the fight, set for Saitama Super Arena, on the northern outskirts of Tokyo.

"It's all about entertainment, come Dec. 31. Mayweather, appearing relaxed and gushing about how he liked Asia and his fans in the region.

Mayweather hasn't fought since beating Conor McGregor more than a year ago for his 50th win from 50 fights.

Nobuyuki Sakakibara, the head of Rizin Fighting Federation, who traveled to the U.S. to woo Mayweather back to the fight, has said it involves the largest payout for an exhibition fight but has not given specifics.

Mayweather previously said he was already making "seven figures" just by talking about it.

Nasukawa, 20, said he wanted people to see his "courage, determination and challenging spirit."

Nasukawa, despite his no-loss record in kickboxing and mixed martial arts, was previously little known outside Japan. He said he enjoyed the sudden international attention.

He acknowledged he didn't understand what the English comments were saying on his Instagram account but added: "I love it."

- AP