Joshiro Maruyama justified his No. 1 seeding by dominating the men's 66-kilogram class at the Dusseldorf Grand Slam on Friday, as Japanese judoka took four of the five gold medals up for grabs on the opening day.

The 25-year-old Maruyama took gold by beating South Korea's Kim Lim Hwan with an uchi-mata inner thigh throw in the final.

(Joshiro Maruyama)

The now five-time Grand Slam winner entered the meet knowing he was the man to beat and came prepared to face opponents who had studied his every move.

"I felt like I had been studied, so I bided my time to find the right moment to counterattack. I knew this was the key to winning," Maruyama said.

In the women's 48-kg class, former world champion Funa Tonaki kept her Tokyo 2020 aspirations on track with an ippon victory by a sumi-gaeshi corner-reversal throw over South Korea's Kang Yu Jeong.

(Tsukasa Yoshida)

Ryuju Nagayama won the men's 68-kg, also deploying an uchi-mata to win the final against Russia's Robert Mshvidobadze.

In the women's 57-kg, reigning world champion Tsukasa Yoshida took gold by beating Olympic champion Rafaela Silva of Brazil with a sumi-gaeshi for a waza-ari score victory.