GUANGZHOU, China -- Marie Bouzkova of the Czech Republic moved into the quarterfinals of the Guangzhou International Women's Open on Thursday after ailing top seed Elina Svitolina of Ukraine retired from their match while trailing 6-4, 4-3, due to right knee pain.

It was an unfortunate early conclusion to a closely contested match, but nevertheless, World No.63 Bouzkova now has three Top 10 wins to her credit this season, adding to her victories over Simona Halep (also via retirement) and Sloane Stephens which came during her semifinal run at Toronto earlier this summer.

"Of course it was a really tough match, even if [Svitolina] wasn’t 100 percent," Bouzkova said, following the encounter. "She was fighting really hard, as much as she could, as much as her body allowed her to."

"It was mentally a tricky match for me, because I knew in the end she’s not feeling her best," Bouzkova continued. "I tried to stay there, stay aggressive, and not give her any easy points."

Photos: Great escapes: Hsieh, Bouzkova join 2019's winners from match point down

Bouzkova was having a superb day on serve, winning 81 percent of points when she got her first delivery into play. World No.3 Svitolina, playing her second event since her semifinal run at the US Open, only got 40 percent of her first serves into play, and fired seven double faults during the tilt, before taking a medical time-out early in the second set which presaged her retirement.

Bouzkova had little trouble on serve in the opening frame, as the Czech won 20 of 24 points in her service games. By contrast, Svitolina needed to save three break points in the early stages of the first set, but the Ukrainian was able to steer to service holds as the combatants stayed level through 4-4.

After a backhand crosscourt winner gave Bouzkova a hold for 5-4, Svitolina became embroiled in another protracted service game. The top seed saw a 40-15 lead slip, and after four deuces, a solid return by Bouzkova led to a netted forehand from Svitolina, giving the Czech set point. There, Bouzkova converted a break point for the first time, blasting a backhand return down the line for a clean winner, and clinching the first set.

Svitolina finally earned her first break of the match in the opening game of the second set, blasting a backhand winner down the line on her third break point of the game. Bouzkova, though, stayed calm, and after holding for 2-1 with an ace, she earned break points in the next game, where Svitolina promptly double faulted to put the set back level.

At the 3-2 changeover, Svitolina received a medical time-out but came back to hold for 3-3. Still, the Ukrainian was struggling with her knee, and after saving two of three break points at 4-3, the top seed felt unable to continue, and stopped the match, sending Bouzkova through to the quarterfinals in China.

Bouzkova had to save four match points in her first-round match against Xun Fang Ying, but has prevailed throughout the week thus far to reach her second WTA quarterfinal of the season. The Czech will face the winner of the late-night match between Russia's Anna Blinkova and Aleksandra Krunic of Serbia in the elite eight.

"I’m going to watch [Blinkova and Krunic] a little bit to see how they play, and I’m excited to play tomorrow," said Bouzkova.

In other second-round results, two-time champion Zhang Shuai of China picked up her first win against Andrea Petkovic of Germany in their four meetings, triumphing 7-5, 6-4. Zhang will face Switzerland's Viktorija Golubic in the quarterfinals, after Golubic upset No.5 seed Katerina Siniakova of the Czech Republic, 4-6, 7-5, 6-0.

2011 US Open champion Samantha Stosur of Australia, a wild card into the field, eked past Bernarda Pera of the United States, 6-4, 1-6, 7-6(5), to reach her second WTA quarterfinal of the year. Former World No.4 Stosur will meet Serbia's Nina Stojanovic, who ousted Chinese wild card Peng Shuai, 7-6(4), 2-6, 6-0.

Sofia Kenin of the United States, now the highest-seeded player left in the draw, also advanced to the quarterfinals, as the No.3 seed beat Ukrainian qualifier Katarina Zavatska, 6-4, 6-2. Kenin will play another qualifier in the next round, Italy's Jasmine Paolini. Paolini upset No.6 seed Zheng Saisai of China, 7-5, 3-6, 7-5, in a two-hour and 42-minute tussle.