The 20-year-old Ratchanok was never in her momentum against the 12th ranked Yamaguchi who mixed up the pace and a variety of shots to dominate the match in 46 minutes.
“I was unable to control my mental condition. I put myself under pressure. I made easy mistakes. I let her play her game,” said Ratchanok.
“Olympics is a big event. Many expect a lot from me. It’s hard to cope with the pressure out there. I hope I can do better in the next Olympics.”
However Porntip Buranaprasertsuk kept the Thai badminton team’s hope alive as she reached the quarter-finals by defeating Maria Ulitina of Ukraine 21-14 21-16 in 39 minutes.
But Porntip has a big hurdle waiting for her in the next round as she is to face defending champion Li Xuerui of China.
Ratchanok’s unexpected loss was another big blow of the Thai Olympics team as several medal hopes including world No 1 skeet shooter Sutiya Jiewchaloemmit and boxer Wuttichai Masuk did not live up to the expectations.