World number one Iga Swiatek was sent crashing out of the US Open on Wednesday by Jessica Pegula who moved into her first Grand Slam semi-final.
Sixth-ranked Pegula triumphed 6-2, 6-4 and will face Karolina Muchova of the Czech Republic for a place in Saturday's final.
If the 30-year-old gets through that she could set-up an all-American final if Emma Navarro stuns world number two Aryna Sabalenka in Thursday's other semi-final.
Victory ended Pegula's run of losing all six Grand Slam quarter-finals in which she had played and extended her streak to 14 wins in 15 matches on US hard courts this summer.
"Finally I can say I'm a semi-finalist. I lost so many of these damn things," said Pegula after her fourth career win against Swiatek.
"Thanks to the crowd. I sent over a 65mph second serve (on a third match point) because I was so tight.
"I did everything I could to not get frustrated. I took advantage of some things she was not doing so well and just rode that momentum."
Pegula raced into a double break in the first set as 2022 US Open winner Swiatek was plagued by mistakes.
{{/usCountry}}Pegula raced into a double break in the first set as 2022 US Open winner Swiatek was plagued by mistakes.
{{/usCountry}}By the end of the opener she had hit just three winners and committed 19 unforced errors as her game fell apart under the Arthur Ashe Stadium lights.
It was a dramatic slump for four-time French Open champion Swiatek who hadn't faced a single break point in her three preceding matches at the tournament.
Pegula turned the screw for a 2-1 lead in the second before Swiatek retrieved the break.
But the Polish star couldn't capitalise and was broken again when a forehand error handed Pegula a 4-3 edge.
The American claimed victory on a third match point when Swiatek went wide again for her 41st and final unforced error of the night.
Unseeded Muchova earlier eased into her second successive US Open semi-final by sweeping past Beatriz Haddad Maia 6-1, 6-4.