The 25-year-old Japanese was stunned by a 14-12, 14-21 straight game defeat in the 27-minute long opening match at the Axiata Arena.

It was only Yamaguchi’s second defeat to the 22-year-old former World Junior Champion in 10 meetings, with the first being their maiden encounter at the 2018 Asian Games team event in Jakarta.

“She was very focused … I still can’t believe I lost this match,” she said through an interpreter.

The Japanese shuttler has yet to win a title since the All England in March, with her best performance being second place at the Asian Championships.

Her opponent Gregoria agreed that she wasn’t on form today.

“Maybe I got the advantage because she failed to give it a fight in the first game, maybe she wasn’t ready. In the second game, I just had a good start.

“I did not set any targets here. I’m just taking a step-by-step approach and trying to play better than my previous tournament,” she said.

Gregoria is set to face the winner of the match between China’s Wang Zhi Yi and He Bing Jiao in the second round.