World number one Ash Barty survived a scare to reach the semi-finals of the Stuttgart Open with a 2-6 6-1 7-5 win over Karolina Pliskova.

The Australian started slowly in the quarter-final of the WTA 500 tournament on Friday as Pliskova broke her serve twice in taking the first set, before Barty hit back with three breaks of her own to force a decider.

Barty was two points away from losing on five occasions as world number nine Pliskova served for the match at 5-4 in the final set.

But she broke the Czech's resolve eventually, sealing victory when Pliskova hit the ball long after a baseline rally.

The win was Barty's eighth consecutive victory over a top-10 opponent and she said in a media conference afterwards that facing top players in the world brings the best out of her.

"When you come up against top-10 opponents, you have to go to that level above to be able to compete. They force you to bring your best," Barty said.

"With a lot of the girls in the top 10, we've had plenty of matches that have gone either way, different experiences, so each time I love that challenge of testing myself against the best."

Barty will face Elina Svitolina in the semi-finals after she saved two match points to claim a 6-7 (4-7) 7-5 6-2 victory over Petra Kvitova.

In the other half of the draw, Simona Halep breezed through to the semi-finals with a 6-1 6-4 win over Ekaterina Alexandrova.

Halep clinically closed out the first set, and although Alexandrova showed more fight in the second, the world number three did not allow her a route back into the match.

Halep will face Aryna Sabalenka following the Belarusian's 7-5 4-6 6-1 win over Anett Kontaveit.

In the Istanbul Cup, top seed Elise Mertens secured her place in the semi-finals with a 6-4 6-4 win over Katerina Siniakova.

Mertens will face Veronika Kudermetova who fought back from a set down to clinch a 2-6 6-3 6-3 win over Ana Bogdan.

The other semi-final will see Sorana Cirstea play Marta Kostyuk. Cirstea progressed after Fiona Ferro, trailing 6-4, retired through injury while Kostyuk beat Ana Konjuh 7-5 4-6 6-3.

Ash Barty made quick work of Laura Siegemund on her Stuttgart Open debut with a 6-0 7-5 triumph in just 84 minutes.

The world number one was given a much sterner test by the former champion in the second set but won all 24 points when landing her first serve and had 19 winners to her name.

Next up for the amiable Australian is a meeting with either Jelena Ostapenko or Karolina Pliskova, who made it through to the round of 16 on Wednesday as did Angelique Kerber and Marketa Vondrousova.

Defending champion Petra Kvitova, seeded sixth, had to overcome a second-set blip to earn a 6-3 3-6 6-3 triumph over Maria Sakkari and a spot in the quarter-finals.

Anett Kontaveit continued her love affair with the tournament, upsetting third seed Sofia Kenin 7-5 6-4. She has now reached the quarter-finals or better in each of her four visits to Stuttgart – including a runner-up finish last season.

At the Istanbul Open, seeded pair Daria Kasatkina (4) and Wang Qiang (8) were ousted by Marta Kostyuk and Ana Konjuh respectively in the round of 16.

Katerina Siniakova and Sorana Cirstea both made the quarter-finals, while Kaia Kanepi was a last-32 victor over Zarina Diyas.

Petra Kvitova brushed aside Jennifer Brady as the defending champion cleared her first hurdle at the Stuttgart Open.

The Czech left-hander was the last winner of the German tournament, beating Anett Kontaveit in the 2019 final, with last year's event cancelled due to the pandemic.

Kvitova was handed a tough opening assignment against Australian Open runner-up Brady, but the American's streak of defeats since that Melbourne final defeat extended to three as Brady lost 6-4 6-3 in an hour and 32 minutes.

Two-time former Wimbledon champion Kvitova, seeded seventh, picked apart the Brady serve once in each set and fended off three break points across the contest to secure her last-16 place.

She was joined in the second round by powerful Belarusian Aryna Sabalenka, as the fifth seed scorched to a 6-2 6-2 win over China's Zhang Shuai.

Earlier, Ekaterina Alexandrova, Kontaveit, Anna Lena Friedsam and Laura Siegemund also went through to the last 16 of the WTA 500 event.

Meanwhile, Fiona Ferro caused a shock at the Istanbul Cup as she landed a 7-5 6-2 first-round win over former champion Petra Martic.

Martic, seeded second this year and ranked 21st in the world, landed the title in Istanbul in 2019 but was rolled over by French player Ferro in an hour and 45 minutes, in a match featuring seven breaks of serve.

Veronika Kudermetova followed her recent maiden title in Charleston with a nerve-jangling first-round win against Bernarda Pera, the fifth seed scrambling through 6-4 3-6 7-6 (7-4).

There was no such toil for top seed Elise Mertens, a 6-2 6-0 winner against Lara Arruabarrena, or for Daria Kasatkina, who dished out a 6-1 6-0 drubbing to Tereza Mrdeza.

Kasatkina is seeking her third title of 2021, having won previously in Melbourne and St Petersburg.

Anastasia Pavlyuchenkova bowed out of the WTA 250 event though, beaten 7-6 (7-2) 6-7 (8-10) 6-4 by compatriot Anastasia Potapova.

Belinda Bencic claimed her first win on clay since the 2019 French Open as she saw off teenager Nastasja Schunk at the Stuttgart Open.

The world number 12 eased to a 6-4 6-2 victory over the 17-year-old qualifier, who saved five match points in her final service game before Bencic closed things out.

"My love affair with clay is... I'm trying to improve myself on it," Bencic said after her first-round triumph.

"I think I'm getting better and I'm taking falls sometimes. I'm trying my best on this surface."

Maria Sakkari was similarly comfortable in progressing to the next round, hitting 24 winners as she dispatched Andrea Petkovic 6-2 6-2 on her debut at the tournament.

At the Istanbul Cup, four of the five scheduled matches were completed before rain stopped play with Anastasia Pavlyuchenkova and Anastasia Potapova level at a set apiece.

Eighth seed Wang Qiang fought back from a set down to beat Anastasia Gasanova 4-6 6-1 6-2.

There were also wins for Ana Konjuh, Katerina Siniakova and Viktorija Golubic.

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