During his ten-year career from 1998 until 2008, he fought a variety of K-1 champions including Ernesto Hoost, Peter Aerts, Remy Bonjasky, Mark Hunt, and Andy Hug.

Losses Stan Longinidis and Andy Hug followed, but Nakasako rebounded with wins over world champion Dewey Cooper and Muay Thai exponent Tofan Pirani.

Despite an eventual TKO loss, Nakasako made a strong showing against K-1 World Grand Prix 2001 winner Mark Hunt at K-1 Rising 2002, scoring a knockdown with a kick to the head and pressuring the champion effectively.

After a dynamic bout wherein the Japanese fighter appropriated his opponent's signature flying knee attacks, Nakasako eventually suffered a TKO in the closing seconds of the final round.

Further tournament competition followed, with Nakasako continuing to fall short but nevertheless achieving the occasional surprising victory, such as his defeat of his rival Kaoklai Kaennorsing at the K-1 World Grand Prix 2006 in Seoul.