The baby-faced southpaw with a blond quiff, who is only a few months out of his teens, was barely known outside his home country before the shock announcement of his New Year's Eve fight with the unbeaten Mayweather.

But the 20-year-old from Chiba near Tokyo is considered something of a prodigy in Japanese fighting circles and began learning karate at the tender age of five.

Like Mayweather, he has never been beaten, boasting a 27-0 record as a professional kickboxer with 20 wins by knockout. He has four wins out of four in mixed martial arts.

Nasukawa won the world junior karate championship when he was a fifth grader and shifted to kickboxing a year later, according to his official website.

He made his professional kickboxing debut at the age of 16 and became bantamweight oriental rules world champion a year later.