Floyd Mayweather knocked down Tenshin Nasukawa three times in the first round as he beat the Japanese kick-boxer in an exhibition match worth a reported $9m.
Former five-weight world champion Mayweather, 41, was smiling as he floored the 20-year-old with ease.
The pair were set to box three rounds of three minutes in Tokyo, but Nasukawa was caught with a left hook before his team threw in the towel.
Despite agreeing to the bout, Mayweather said he was “still retired”.
“It was all about entertainment – we had a lot of fun,” said Mayweather, who beat UFC’s Conor McGregor in a boxing match in August 2017. “They wanted this to happen in Japan, so I said ‘why not?’”
BBC reports that both fighters were undefeated coming into the contest, which was delayed by several hours.
After beating Nasukawa in the exhibition match, Mayweather insisted: “I’m still undefeated; Tenshin is still undefeated.”
Addressing the Japanese fighter, he added: “Hold your head up high.”