Suriyan Sor Rungvisai lost his WBC super-flyweight title to Japanese opponent Yota Sato in Tokyo yesterday.
In only his second defence, the Thai was outscored by the No.3 challenger who was successful in his first shot at the world title, winning by a unanimous verdict, 114-112, 116-110, 114-112, after 12 rounds.
The turning point came in the third round when Suriyan was overpowered and floored twice.
The champion then tried to move forward but was kept at bay by Sato who is 10cm taller than the Thai and impressively used his height advantage.
Suriyan, who wept after the defeat, said he lost because of his poor showing in the third round.
"After the third round, I tried to do everything but nothing worked," said the Nakhon Ratchasima fighter.
"He gave me a big boxing lesson and I will learn from it."
Sato said his hard work had paid off handsomely and he dedicated the win to the people of his hometown who were hit by the terrible tsunami that struck last year.
"We studied his style thouroughly and that was why I won today," he said.
Suriyan won the title last year when he dethroned Tomas Rojas of Mexico and then beat Japan's Nobuo Nashiro in his first defence in Bangkok in November.
His record dropped to five losses against 20 wins and one draw, while the new champion registered his 24th win with two defeats and one draw.
Suriyan's manager Surachart Pisitwutthinan admitted his man had lost to a better fighter.
However, the Thai holds two options and is assured of a rematch.
Suriyan is the second Thai to have lost a world title this month after WBC flyweight champion Pongsaklek Wonjongkam was overwhelmed and stopped by Sonny Boy Jaro of the Philippines in Chon Buri.
Currently, Thailand has only two world boxing champions; WBC light-flyweight title-holder Kampayak Por Pramuk and WBA super-flyweight king Tepparith Singwangcha.