Mayweather knocked him down three times in two minutes, eventually forcing Nasukawa's corner to throw in the towel.
Mayweather, 41, a former five-weight world champion who retired from boxing with a flawless professional record of 50-0 last year, saw off an opponent 21 years his junior.
So, when Mayweather was approached for a post-fight interview, the often-controversial fighter had the ideal opportunity to brag about his performance.
Although the injury was certainly a grim talking-point, it was, of course, Mayweather versus Nasukawa that had everyone talking.
"We are both still undefeated", Mayweather said, per The Mirror's Jake Polden. Mayweather toyed with him throughout.
What did you think of the exhibition fight between Tenshin Nasukawa and Floyd Mayweather? I'm still retired, still 50-0. Toyko, Japan - you guys are fantastic - I want to say "Thank you". I want to thank God for this event, for my team.
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The American said in quotes published by BBC Sport: "It was all about entertainment - we had a lot of fun". I'm still retired and I don't look forward to coming back to boxing.
But Nasukawa never landed a meaningful punch.
But Mayweather changed his tune after knocking out Nasukawa, who is 27-0 with 21 knockouts in kickboxing and 4-0 with two knockouts in MMA. "I'm still undefeated. This was just entertainment for the people". He's still a true champion. He's a great guy, he's a guy fighter.
Addressing the Japanese fighter, he added: "Hold your head up high". "He is a great fighter and a great champion".
The man they call "Money" was paid an obscene £$9 million for what turned out to be less than three minutes' work - not bad, eh?