
The 2017 world junior champion stunned top seed and world No 1 Yamaguchi 25-23, 15-21, 21-10.
This is the second time in a row Gregoria has defeated Yamaguchi after winning 21-14, 21-14 in the first round of the Malaysia Open last week.
Unseeded Gregoria said her semifinal against third seed An Se-young tomorrow will also provide an opportunity to avenge a previous loss to the South Korean.
Se-young eliminated her in the first round of the All England in Birmingham in March.
“I lost to her in the All England, but I will try my best to win tomorrow,” she said.
Commenting on today’s action, Gregoria said she was able to capitalise on Yamaguchi’s recent loss of form.
Meanwhile, Yamaguchi said she was not unnerved by her exit from the Malaysia Masters.
After her shock early exit in the Malaysia Open last week, she was pleased to do better by checking into the last eight.