[3] Yonekura won the All-Japan Amateur Boxing Championships in the flyweight division in 1956,[4] and represented Japan at the 1956 Summer Olympics in Melbourne, while studying at Meiji University.
[6] Before his official debut, he made an exhibition match against the Japanese flyweight champion Sadao Yaoita as a welcome bout for the NBA's public relations manager.
[7] His first world title shot against Pascual Pérez in the flyweight division, which was postponed three days due to rain,[8] ended in a unanimous decision loss after being knocked down in the second round, in front of 9,000 spectators at the Tokyo Metropolitan Gymnasium in August 1959.
In his second world title shot in the bantamweight division in May of that year, he lost to José Becerra via a split decision[13] while being watched by 17,000 spectators at the Korakuen Baseball Stadium.
[7] After his retirement as a boxer, he opened Yonekura Boxing Gym in Tokyo in 1963, and acted as its president, promoter, manager, and trainer.