Top seed Samantha Stosur's WTA Citi Open campaign came to a surprising end at the quarter-final stage, as Dominika Cibulkova and Johanna Konta set-up a final-four showdown at the Bank of the West Classic.
Stosur was sensationally eliminated by American wildcard Jessica Pegula 7-6 (7-4) 6-3 on Friday.
Winner of the US Open in 2011, Stosur looked on track to reach the Washington semis when she raced out to a 4-1 lead in the first-set tie-break, only to lost six consecutive points as Pegula moved ahead in the match.
Pegula, whose parents own NFL franchise the Buffalo Bills as well as the Buffalo Sabres in the NHL, broke for a 5-3 lead in the second and she successfully served out the match to earn a semi-final showdown with countrywoman Lauren Davis.
Davis accounted for Camila Giorgi 6-4 6-4.
In a battle of the seeds, Belgian seventh seed Yanina Wickmayer prevailed against fourth-seeded Frenchwoman Kristina Mladenovic 4-6 6-3 6-4.
Yulia Putintseva - the sixth seed - saw off Risa Ozaki 6-4 6-2.
Bank of the West Classic's top seed Venus Williams booked her spot in the final four.
Williams was a class above American compatriot Catherine Bellis - 19 years her junior - winning 6-4 6-1.
Next up for Williams is Alison Riske, who won the opening set 6-3 and was trailing 1-0 in the second when fellow American and fourth seed Coco Vandeweghe retired due to an ankle injury.
Dominika Cibulkova and Johanna Konta, meanwhile, will meet in the semi-finals.
Seeded second, Cibulkova defeated fellow seed Misaki Doi 7-5 6-0 to ensure she returns to the WTA's top 10 for the first time since January last year.
Cibulkova had trailed 5-2 in the first set before winning 11 consecutive games in Stanford.
As for British third seed Konta, she overcame Zheng Saisai 6-2 4-6 6-3.