Yamaguchi held the WBA light flyweight championship of the world, defeating Panamanian champion Carlos Murillo in 1996 via a 12th-round split decision.
[1] After he had won the world championship, he had only defended the title once by defeating his old rival Carlos Murillo in the middle of the same year via a 12th-round unanimous decision.
Later on, he was suddenly lost in the end of the year 1996, when he lost by TKO just the second round against the Thai challenger Pichit Chor Siriwat (previously known as Pichitnoi Sitbangprachan) in a bout at Osaka Prefectural Gymnasium, Osaka.
In 1995, he challenged for the first world championship against Choi Hi-yong, a South Korean crown holder, but he was defeated via a 12th-round split decision.
His nicknamed "Prince" because his idol is Naseem Hamed, a superstar British boxer of Yemeni descent in featherweight class.