Katie Volynets has reached her first WTA semi-final by taking big leads in each set and then fending off reigning NCAA singles champion Peyton Stearns to win 7-5 6-3 at the ATX Open in Austin.

The 92-ranked Volynets went ahead 4-0 in the opening set, then found herself at 5-all. But she managed to run off seven games in a row from there to claim that set and lead 5-0 in the second.

Once again, things got a bit complicated against Stearns, who collected her first two WTA match wins this week.

Stearns, a wild-card entry, saved one match point while Volynets served for the victory at 5-0 in the second, then broke at love when Volynets served for the match again at 5-2.

In the last game, Volynets needed three more match points to finally seal the victory, using a big forehand on the 12th stroke of a baseline exchange to draw a backhand that landed long.

Volynets was making her WTA quarter-final debut and got there by saving a match point and erasing a 5-0 deficit in the third set against third-seeded Anastasia Potapova on Wednesday.

She did not want to be on the opposite end of that sort of comeback on Friday.

"I just was telling myself to keep going and to just keep playing my game. And sometimes it wasn't happening," said Volynets, who advanced to the third round of the Australian Open in January.

"But when someone goes down (by) a big lead, sometimes there's nothing to lose anymore. It happened to me a couple of days ago, actually. So I was just trying to stay tough."

She faces 2017 US Open champion Sloane Stephens or Varvara Gracheva next.

Earlier Friday, eighth-seeded Marta Kostyuk made it to the final four of a tournament for the first time in 2023 by defeating Anna-Lena Friedsam 7-6 (8-6) 6-2.

Kostyuk overcame nine double-faults and got broken four times.

She entered the day 0-2 in quarter-finals this season but will now try to reach the first WTA final of her career when she meets 2022 Australian Open runner-up Danielle Collins.

The fourth-seeded Collins grabbed 11 of the last 12 games to come back and eliminate Anna Kalinskaya 3-6 6-2 6-1 in the quarter-finals.