Sloane Stephens became the latest seed to be knocked out at the ATX Open in Austin after suffering a straight-sets defeat to Varvara Gracheva in the quarter-finals on Friday.

Gracheva won 7-6 (7-4) 6-3 over fifth seed Stephens in one hour and 53 minutes, with the Russian generating 11 break points throughout the match.

The victory means world number 88 Gracheva adds Stephens to top seed Magda Linette to her impressive list of scalps this week in Austin.

Fourth seed Danielle Collins moved into favouritism with a 3-6 6-2 6-1 victory over Ann Kalinskaya, rallying back from a set down to win in one hour and 49 minutes.

Collins will face eighth seed Maria Kostyuk in the semi-finals, with the Ukrainian defeating Anna-Lena Friedsam 7-6 (8-6) 6-2.

Gracheva will take on Katie Volynets who ended Texas native Peyton Stearns' run, winning 7-5 6-3.

Top seed Carolina Garcia eased into the Monterrey Open semi-finals with a 6-0 6-4 victory over seventh seed Mayar Sherif.

Garcia will face Elise Mertens in the semi-final after the Belgian knocked off Elisabetta Cocciaretto 6-2 6-2.

On the other side of the draw, third seed Donna Vekic got past Ysaline Bonaventure 6-2 5-7 7-6 (7-5) in two hours and 15 minutes, while fifth seed Zhu Lin won 6-3 6-2 over American qualifier Caroline Dolehide 6-3 6-2.