The rules were very strictly defined: three, three minute bouts and boxing only - kickboxer Nasukawa was reportedly facing a $5 million fine if he aimed a kick at Mayweather. However, Nobuyuki Sakakibara, the head of Rizin Fighting Federation, which is putting on the three-round exhibition, told reporters that "a misunderstanding" was behind Mayweather, 41, appearing to back out.
Nasukawa, who is an undefeated kickboxer and MMA fighter, started brightly but dropped to the canvas following a sharp right uppercut from Mayweather.
Nasukawa's corner threw in the towel after the third knockdown, and the 20-year-old was left in tears after being humbled by the retired Mayweather.
Afterwards Mayweather paid tribute to Tenshin and said he can come again with his youth and that not to worry as the fight will not go on his or his own professional fight record.
Mayweather got $9 million for the match. It should be noted that the match was an exhibition. Tokyo, I want to say thank you. Tenshin is still undefeated - I'm still undefeated. I did it just to entertain fans in Japan.
However, "Money" didn't need nine minutes to collect his pay cheque, dispatching the Japanese sporting hero with a barrage of combinations in the opening round.
Mayweather, a legend in the boxing world, has a ideal 50-0 record with 27 knockouts. Last year, he knocked out mixed martial arts fighter Conor McGregor in the 10th round of a super-hyped boxing match.