Petra Kvitova whips a forehand to Angelique Kerber at the Sydney International on Thursday. Photo: Rick Rycroft/AP

SYDNEY – Defending champion Angelique Kerber was booted out of the Sydney International by Petra Kvitova in the quarter-finals on Thursday, defeated in straight sets 6-4 6-1.

Despite the German world number two breaking to an early 4-2 lead in the first set, Kvitova soon broke back and, firing 25 winners in total to Kerber’s nine, dominated the rest of the match.

In the semi-final, the two-time Wimbledon champion now faces 24-year-old Belarusian Aliaksandra Sasnovich, who stunned Kvitova in the first round of that tournament last year.

“I know I do have a tough opponent tomorrow, I lost (to) her in Wimbledon... so it will be a tough one, so I’m not really thinking too much ahead,” Kvitova, currently ranked 8, said.

Earlier on Thursday, Australia’s Ash Barty downed Elise Mertens to get through to the semi-finals, while men’s number one seed Stefanos Tsitsipas was dumped out by Italy’s Andreas Seppi.

Barty, who reached last year’s finals, took control of the match as she beat the Belgian world number 12 in straight sets 6-3 6-3 to set up a semi-final with Kiki Bertens of the Netherlands.

“I’m pretty happy with the way I was able to come out here and play nice and solid, served well, I thought I gave myself a chance on return games,” said the 22-year-old, ranked 15, who on Wednesday defeated world number one Simona Halep.

Bertens, who defeated Yulia Putintseva 6-2 6-2, said she didn’t have “great memories” of her last match against Barty, in which the Australian powered past her at the same stage of the Montreal WTA tournament in August.

“She’s a great player,” Bertens told reporters. ”She plays really tricky. I think in Montreal I didn’t play my best, so I’m really excited to play a good match tomorrow.”

Tsitsipas, the 20-year-old Greek who has been touted as a potential future Grand Slam champion, meanwhile failed to take advantage of his first set win and went down to Seppi 4-6 6-4 6-4.

Seppi, ranked more than 20 places below Tsitsipas at 37, will face Diego Schwartzman in the semi-final after the Argentinian took out Japan’s Yoshihito Nishioka, 1-6 6-3 7-6 (7-1).

Home favourite Alex de Minaur, who also reached last year’s final, beat fellow Australian Jordan Thompson in a close-fought match in which he prevailed 7-6 (7-4) 6-3.

“It’s always really tough to play a mate and a guy that I’ve grown up with and pretty much seen as a big brother to me,” said 19-year-old de Minaur. “I’m very happy that I managed to get the win.”

He will face fourth seed Gilles Simon of France, who outlasted John Millman 7-5 3-6 7-6 (7-3) after the Australian failed to serve out the match at 6-5.