At the time of the fall of the ancient Siam capital of Ayutthaya in 1767, the invading Burmese troops rounded up a group of Thai residents and took them as prisoners. Among them were a large number of Thai boxers, who were taken by the Burmese to the city of Ungwa.

In 1774, in the Burmese city of Rangoon, the king of the Burmese, King Mangra decided to organize a seven-day, seven-night religious festival in honor of Buddha's relics. The festivities included many forms of entertainment, such as the costume plays called likay, comedies and farces, and sword-fighting matches. At one point, King Mangra wanted to see how Muay Boran would compare to the Burmese art Lethwei. Nai Khanom Tom was selected to fight against the Burmese champion. The boxing ring was set up in front of the throne and Nai Khanom Tom did a traditional Wai Kru pre-fight dance, to pay his respects to the Burmese king, as well as for all the spectators, dancing around his opponent, which amazed and perplexed all the Burmese people. When the fight began, he charged out, using punches, kicks, elbows, and knees, pummeling his opponent until he collapsed.[2]

The referee however stated that the Burmese opponent was too distracted by the Wai Kru, and the knockout was invalid. The King then asked if Nai Khanom Tom would fight nine other Burmese champions to prove himself. He agreed and fought them all, one after the other with no rest periods in between. His last opponent was a great boxing teacher from Ya Kai City, Nai Khanom Tom mangled him by his kicks and no one else dared to challenge him any further.

King Mangra was so impressed that he remarked, "Every part of the Thai is blessed with venom. Even with his bare hands, he can fell nine or ten opponents. But his Lord was incompetent and lost the country to the enemy. If he would have been any good, there was no way the City of Ayutthaya would ever have fallen."

King Mangra granted Nai Khanom Tom freedom along with either riches or two beautiful Burmese wives. Nai Khanom Tom chose the wives as he said that money was easier to find. He then departed with his wives for Siam. Other variations of this story had him also winning the release of his fellow Thai prisoners. His feat is celebrated every March 17 as "Boxer's Day" or "National Muay Thai Day" in his honor and that of Muay Thai's.

Today, some have wrongly attributed the legend of "Nai Khanom Tom" to King Naresuan, who was once taken by the Burmese. However, Nai Khanon Tom and King Naresuan were almost two centuries apart.