Christina McHale beat Heather Watson 4-6, 6-0, 7-6.
British number two Heather Watson earned her best win of the year to beat second seed Caroline Wozniacki 7-5 6-4 in the Monterrey Open quarter-finals.
Watson broke the former world number one in the penultimate game of the first set in Mexico and sealed victory when the Dane hit a forehand wide.
The 23-year-old plays France's Caroline Garcia (ranked 38) in Saturday's semi-final. Heather Watson has beaten her on the two times they've played.
Earlier, fellow Britain Johanna Konta lost 6-3 6-7 (2-7) 6-1 to world number 73 Kirsten Flipkens.
Watson has slipped down the rankings to number 84 (BBC article is wrong with 70) this year and Saturday's encounter with Garcia will be her first semi-final since winning the WTA event in Hobart in January 2015.
"I thought I played very well," said Watson. "I needed to against Caroline because she's a great player - and I had to fight to the end because I knew she would."
Konta, ranked 27 in the world, was a set and a break down but broke back to level the second set at 5-5 before winning it in the tie-break.
However, she won only one more game as Belgium's Flipkens reached the last four.