Top seed Daria Gavrilova suffered a quarter-final exit at the San Antonio Open as Alison Riske booked her spot in the last four.
Australian Gavrilova, the top seed at the WTA 125K series event, went down to Riske 6-4 6-4 in one hour, 32 minutes on Thursday.
Riske broke five times as Gavrilova struggled on serve, the Russia-born right-hander serving at just 58 per cent with six double faults.
With her win, Riske set up a clash against Anna-Lena Friedsam in the semi-finals after the German battled past Ana Konjuh 6-4 4-6 6-2.
Bulgarian Tsvetana Pironkova cruised into the last four, thrashing Konjuh's Croatian countrywoman Donna Vekic 6-2 6-1.
The only seed left in the draw is Misaki Doi, who was a 6-3 6-3 winner against Samantha Crawford.