Four-time Grand Slam champion Iga Swiatek crashed out of the quarter-finals at the Pan Pacific Open in Tokyo on Friday, while Jessica Pegula and Maria Sakkari stormed into the semi-finals.
Russia's Veronika Kudermetova, ranked 19th, served four aces and beat world number two Swiatek 6-2, 2-6, 6-4 in 2 hours and 21 minutes.
"My service worked really well today," the 26-year-old said after the match.
"In the last game, I tried to focus that I need to play point, I don't need to hit the ace and I just need to put the serve in and try to play a rally."
Kudermetova will face Anastasia Pavlyuchenkova, ranked 86th, on Saturday.
"It'll be an interesting and a very tough match," Kudermetova said.
"I played worse," said Swiatek, who lost her number-one ranking at the US Open.
"I know the mistakes I made... I know exactly how I should play differently so I'm not going to make these mistakes again," she said.
World number four Pegula of the United States beat 26-year-old Russian Daria Kasatkina, ranked 13th, 6-1, 6-0.
"I thought I played a pretty flawless match. Played smart and played aggressive," Pegula said.
She will face world number six Maria Sakkari in the semi-finals, who beat number 10 Caroline Garcia 6-2, 6-2.
Greece's Sakkari said she has played against Pegula many times and they know each other's game really well.
"We never had an easy match. She's very solid and she's a fighter," the 28-year-old said.
"It can be very tough but I'm going to keep doing what I've been doing."
Sakkari ended her four-year wait for a second WTA title at the Guadalajara Open last week, where she beat Garcia in the semi-finals en route to lifting the trophy.
She had come up empty in six finals since her breakthrough victory in Rabat in 2019.
That included the Guadalajara final last year, when she lost to Jessica Pegula, and a loss to US Open champion Coco Gauff in the final of the Washington Open this year.
The tournament concludes on Sunday.