Somluck, then 23, returned from the Olympics a national hero. He had an audience with King Bhumibol Adulyadej. He received more than $1 million in bonuses from the government and his sponsors. “My life went from the earth to the stars,” he said. “I was famous. I was able to do advertising, movies, music, you name it. Everything changed.”

Somluck is the Muhammad Ali of Thailand, but few people outside his homeland saw his movies, “Born to Fight” (2004) and “Soi Cowboy” (2008), or his debut as a singer in 2006. His appearances in the ring decreased as his celebrity increased, but he has defeated foreigners in sporadic muay Thai matches. Somluck has continued to box, losing to the American Rocky Juarez in the quarterfinals at the 2000 Olympics in Sydney, Australia, and falling in the first round at the 2004 Games in Athens. In the early 2000s, he opened a muay Thai gym in Bangkok.

Muay Thai’s popularity has grown with recent international competitions like Thai Fight and Best of Siam, and in mixed martial arts organizations like the Ultimate Fighting Championship and the K1 circuit, with their corner girls, light shows and fireworks.

But Thais under 30 are increasingly more interested in English Premier League soccer than their national sport. Widespread mobile phone use has enabled gamblers to bet on muay Thai fights remotely, and attendance at Lumpini and Rajadamnern has fallen.

Somluck noticed other changes in the sport.

“Referees score fights differently these days,” he said. “A boxer can have the better kicks but lose because he gets thrown down in the clinch. Fighters don’t use classical muay Thai anymore; they spend most of the time clinching like wrestlers. It’s made people bored of watching the fights. The art is gone. If people want to see real art muay Thai now, it’s better to watch foreigners.”

Techniques that were once part of Somluck’s repertory — the spinning back kick, known as the crocodile tail whip, and the elbow smash to the thigh, or breaking the elephant’s trunk — are now rarely seen in Thai stadiums.