Qualifier Emma Navarro of the United States upset world number nine Maria Sakkari of Greece, winning in a third-set tie-break, to reach the semi-finals of the WTA's San Diego Open on Thursday.
On a strong day for Americans, former Australian Open champion Sofia Kenin and Danielle Collins also advanced to the last four.
Kenin will meet 61st-ranked Navarro on Friday while Collins will be up against fourth-seeded Barbora Krejcikova of the Czech Republic, who triumphed against Brazil's Beatriz Haddad Maia.
The 22-year-old Navarro won 6-4, 0-6, 7-6 (7-4) against Sakkari in two hours and 13 minutes.
It was the New Yorker's first win against a top-10 player and continued a good tournament for the 2021 NCAA singles champion, who went into the match having not lost a set in qualifying or the main draw.
Sakkari broke first to lead 3-1 in the opening set but then struggled with Navarro's powerful baseline play as the American fought back to take the set.
The Greek dominated the second but Navarro showed plenty of fight to break first in the third set to take a 3-2 lead, only for Sakkari to instantly break back.
In the tie-break Navarro was quick out of the blocks, opening up a 4-0 advantage and never looking back.
Kenin, who captured her first Grand Slam title in 2020 at Melbourne, dispatched 27th-ranked Anastasia Potapova 6-2, 6-3.
Potapova made 15 double faults and landed only 46.4 percent of her first serves to fall in 78 minutes.
World number 93 Kenin reached her first WTA semi-final since Hobart in January.
"I'm super happy, I played really well. Good matches under my belt definitely gives me a lot of confidence, so I'm doing everything right," said Moscow-born American Kenin.
"My mindset is there. I'm feisty, energetic, and it's all good," she added.
Collins, who lost to Ash Barty in the 2022 Australian Open final, eliminated France's 10th-ranked Caroline Garcia 6-2, 6-3, in only 67 minutes.
"I felt like I was really feeling my shots and my game, so I was really in a good headspace and flowing the whole match," said Collins.
Collins, ranked 43rd, will have a tough task against Krejcikova after she defeated seventh seed Haddad Maia 6-4, 6-3.
Krejcikova, the French Open winner in 2021, hit 24 winners to the Brazilian's eight and conceded she had little idea of what kind of opponent Collins will be.
"I never played her, I never practiced with her, so I have no idea what to expect," she said.