Yota Sato lost his WBC super flyweight title by technical knockout to challenger Srisaket Sor Rungvisai on Friday.
Sato, making his third title defense, lacked energy as the fight wore on, while Srisaket punched aggressively and took control of the fight, which was stopped in the eighth round.
"I was in perfect condition up until the fight, but I was unable to fight as well as I can," Sato said. "I punched but he never drew back. He was strong."
"In the latter half of the fight, I tried to look for a spot to turn things around, but I was just too exhausted."
Srisaket, 26, improved to 19-3-1, with 18 wins coming by knockout. The 29-year-old Sato slipped to 26-3-1 (12 KOs).
According to the fight's organizer, Japanese male fighters are 0-17 with one tie in world title bouts held in Thailand.