World No 4 Jessica Pegula rallied from a set and a break down to beat defending champion Danielle Collins 1-6, 6-3, 6-0 on Friday and reach the semifinals of he WTA clay court tournament in Charleston, South Carolina.
"That was tough," said Pegula, who trailed 6-1, 2-0 but won 10 of the last 13 games – including the last nine to close out the contest.
"Obviously she can play well here," Pegula said. "She's defending champion and she came out firing. I was not ready for that at all.
"Luckily I ws able to dig my heels into that second set and play some good tennis."
Pegula said the key to turning things around was mixing up her shots and "smart serving".
"She was returning unbelievable, just hitting winner, winner, winner off my serve and I had to figure out a way to get my placement a little bit better," Pegula said.
Pegula, the top seed, booked a semifinal meeting with Russian Ekaterina Alexandrova, who beat Paris Olympics gold medallist Zheng Qinwen of China 6-1, 6-4.
Alexandrova reached her third semifinal of the season with her third win in three meetings with Zheng, the third seed.
Alexandrova saved all three break points she faced and converted three of her five break points against the Chinese star, who hit just eight winners and had treatment on her right arm during the first set.
American Sofia Kenin, the 2020 Australian Open champion, also reached the semifinals, beating Russian Anna Kalinskaya 6-4, 6-3.
Kenin, who is through to the semis for the first time since Tokyo last year, will next face American eighth seed Amanda Anisimova in the last four. Anisimova advanced with a 7-5, 7-6 (7/1) defeat of fourth seed Emma Navarro.