It has already been described as boxing's biggest mismatch - fixed even - and Floyd Mayweather's surprise admission shortly after his crushing victory over Tenshin Nasukawa will do little to disprove that.

Mayweather, 41, admitted he wasn't "in shape" in the bizarre exhibition bout in Japan, that lasted only 136 seconds - with the undefeated former world champion flooring the little-known Japanese mixed martial artist three times in the first round.

Yet "Money", one of Mayweather's many monikers, pocketed a staggering $13 million.

Many boxing fans have taken to social media to slam the event with some calling it rigged.

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Mayweather responded with a post on Instagram: "What if I told you I was making $9,000,000 for 9 minutes of sparring in Tokyo Japan.

"Would you do the same if you were me? I like to call it a 9 minute walk through."

He did, however, that he was far from prepared for the fight.

"You know me, I'm just an old man. I ain't in shape, body ain't in shape, I'm just an old man trying to make it," Mayweather said.

Mayweather, who has a perfect 50-0 professional record and who last fought in August 2017 when he beat UFC superstar Conor McGregor in boxing's "richest bout", insisted that he won't come out of retirement.

For a third time.

"It was all about entertainment but we had fun," he said.

"Tokyo, Japan, you guys have been amazing and thank you. I want to say thank you to Tenshin. This is all about entertainment. Tenshin is still undefeated, I'm still retired.

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"I did this just to entertain. Once again, I'm still retired. I'm still 50-0."

"I'm going to say it again, I'm still retired — I don't look forward to coming back."

The 20-year-old Nasukawa insisted afterward that he didn't regret taking the fight.

"It was a challenge to fight with being said to be reckless. Although the result has been knocked down, I do not regret it.

"I did not reach Mayweather at my own 20 years, but I came out with courage.

"Thank you very much, Mayweather players who fought, thank you very much for everyone who supported me.

"I could not shape your thoughts, but I will continue to challenge from now on — everyone, thank you again."