Coco Vandeweghe ended Karolina Pliskova's reign as world number one and made it three American women in the US Open semi-finals with a straight-set win.
The 20th seed won 7-6 (7-4) 6-3 in the last eight at Flushing Meadows.
Vandeweghe, 25, goes on to face compatriot Madison Keys or Latvia's Anastasija Sevastova in the last four.
A Keys victory would create the first all-American semi-final line-up at the US Open since 1981.
Two-time champion Venus Williams and unseeded Sloane Stephens are already through on the other side of the draw.
Pliskova, 25, needed to reach the final to hang on to the number one ranking, but defeat means she will be replaced by Spain's Garbine Muguruza in the new standings next week.