Former five-weight world champion Mayweather, 41, was smiling as he floored the 20-year-old easily.
Mayweather lands another left hook that sends Nasukawa off balance.
"Another day like I wake up, take shower, put my clothes on", Mayweathersaid.
"It was all about entertainment". I am still retired, I don't look to come back to boxing. "The fans in Japan, they wanted this to happen, so I said why not. I'm still retired. I'm still 50-0 and Nasukawa is still undefeated, and a hell of a fighter". Mayweather toyed with him throughout.
The size difference was readily apparent between the two as Mayweather was the taller, bigger, and longer boxer.
As for Mayweather - don't be surprised if he picks up another one of these easy pay day boxing matches in the new year. Subsequently, after his corner threw in the towel following Mayweather's trio of first round blows, Nasukawa was left in tears.
It was an unexpected turn of events, and the rule set was even more freaky.
Mayweather was late to arriving at the Saitama Super Arena in Saitama, Japan and the co-main event and main event of the Rizin card was delayed until his arrival.
The contest was eventually waved off after Nasukawa was floored on three occasions, with the final knockdown leaving him flailing against the ropes.
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. Nasukawa, however, barely landed a clean punch against the vastly more experienced Mayweather.
Mayweather began smiling and relaxed.
Nasukawa was a heavy underdog in the fight, which wasn't even listed by any organization as an official bout.