Norifumi “Kid” Yamamoto, a fighter who was the biggest MMA superstar in Japan during the mid-2000s, passed away on Tuesday at the age of 41.

Yamamoto had announced on Instagram on Aug. 21 that he was battling cancer. He was hospitalized and receiving treatment in Guam at the time of his death.

Yamamoto, even despite being just 5-foot-3 and 141 pounds, competed mostly as a lightweight and sometimes as a featherweight against opponents who dwarfed him throughout his career. He compiled a 17-1 record at the time he retired for the first time following a 2007 New Year’s Eve win over Rani Yahya in Osaka.

Yamamoto for a time was a rock star level celebrity in Japan with major commercial endorsements. He was the youngest son of a famous Japanese wrestling family. His father, Ikuei, represented Japan in the 1972 Olympics in Munich, Germany. His older sister Miyuu, who was named after the city of Munich, won three world championships in wrestling, the first at the age of 17. Another older sister, Seiko, won four world championships in wrestling.

However, the heyday of Ikuei Yamamoto’s daughters was prior to women’s wrestling being an Olympic sport.

Known as Kid Yamamoto, as the youngest child in the family, Yamamoto was sent by his father to Arizona to go to high school and train with Townsend Saunders, an Olympic silver medalist in wrestling. Yamamoto won three state titles in Arizona, but left the sport at the age of 21 to pursue fighting. He was trained at first by Enson Inoue, a Hawaiian who was a top MMA star in Japan, who at the time was married to Miyuu.

His father at first looked down on MMA, not considering it a real sport. He had pushed for his youngest son to return to wrestling and become the family’s first Olympic gold medalist. Kid Yamamoto shocked the fighting world when he announced in 2007, after nine years away from wrestling, that he would be retiring from fighting to pursue a freestyle wrestling gold medal in the 2008 Olympics. His first retirement took place before nearly 40,000 fans at the Osaka Dome, where he defeated current UFC fighter Rani Yahya.

His attempt to fulfill his father’s dream was a major mainstream sports story in Japan, and brought a level of attention to amateur wrestling that the sport has rarely received in Japan. But while competing in the 60 kilogram (132-pound) weight class, he suffered a dislocated elbow after being thrown by Kenji Inoue — who had won a bronze medal in the 2004 Olympics — in the Olympic qualifying tournament. Yamamoto later needed reconstructive surgery for a torn ACL.

Yamamoto returned to fighting in 2009, but was never the same. Even despite competing against bantamweights and featherweights in his comeback, he only won one of his last seven fights. He arrived at the UFC with a lot of hype in 2011, and lost a decision to Demetrious Johnson in his first fight with the organization. He last fought in 2015 and ended his career with an 18-6 record with two no contests.

Yamamoto’s fame exploded when he competed in a December 31, 2004 kickboxing match against Masato, which did a 31.6 rating and more than 34 million viewers. While Yamamoto lost the fight, which drew 35,000 fans to the Osaka Dome, he was giving up probably 20 pounds or more and competing against one of the best kickboxers in the world at the time, and he still held his own and scored an early knockdown.

Yamamoto became Japan’s biggest MMA star in 2005, when he won the Hero’s lightweight championship in a tournament where he defeated Royler Gracie, Caol Uno, and Genki Sudo, all by knockout. He followed that up with a four-second knockout win via flying knee against Olympic wrestler Kazuyuki Miyata, and also defeated 2004 Olympic gold medalist wrestler Istvan Majoros on the 2006 New Year’s Eve show via first-round knockout.