Former Wimbledon champion Petra Kvitova into semifinals at Sydney International

89cfec19-Australia Tennis Sydney International

Petra Kvitova of the Czech Republic plays a shot to fellow countrywoman Lucie Safarova during their women's singles match at the Sydney International tennis tournament in Sydney, Wednesday, Jan. 8, 2014. (AP Photo/Rick Rycroft) (The Associated Press)

6ce387b5-Australia Tennis Sydney International

Petra Kvitova of the Czech Republic hits a backhand to Christina McHale of the U.S. during their women's singles match at the Sydney International tennis tournament in Sydney, Australia, Tuesday, Jan. 7, 2014. (AP Photo/Daniel Munoz) (The Associated Press)

SYDNEY – Former Wimbledon champion Petra Kvitova advanced to the Sydney International semifinals on Wednesday with a 7-6 (4), 6-2 victory over fellow Czech player Lucie Safarova.

Kvitova, the second seed, will play the winner of the quarterfinal being played later Wednesday between Italy's Sara Errani and Bulgaria's Tsvetana Pironkova.

In other women's play at the Australian Open warm-up tournament, defending champion Elena Vesnina retired from her second-round match at the Hobart International with a back injury. Spanish qualifier Estrella Cabeza Candela was leading the match 4-6, 6-2, 4-1 when Vesnina quit.

Vesnina claimed her first WTA title in Hobart last year and was looking to become the first back-to-back winner in the event's 21-year history.

In men's play at Sydney, Australian wild card Marinko Matosevic upset third-seeded Andreas Seppi of Italy 6-3, 6-4 to reach the quarterfinals. Defending champion Bernard Tomic was scheduled to play his second-round match later Wednesday.

At the ATP's Heineken Open in Auckland, New Zealand, American wild card Jack Sock got the biggest win of his career when he beat second-seeded Tommy Hass 6-4, 6-4 in the second round.

The 21-year-old Sock is 14 years younger than Haas and his ranking is nearly 90 places lower than the 12th-ranked German. But Sock started strongly and got early breaks in both sets to pull off the upset.

Sock will face Roberto Batista Agut in Thursday's quarterfinals after the Spaniard beat sixth-seeded Benoit Paire of France 6-3, 6-4.

Third-seeded American John Isner also progressed to the quarterfinals with a 7-6 (1), 3-6, 6-2 win over Lucas Lacko of Slovakia. Isner's participation in the tournament had been in doubt because of an ankle injury, but he seemed fit in a match that lasted nearly two hours.

American qualifier Steve Johnson beat Cypriot Marcos Baghdatis 7-5, 6-7 (3), 6-4 in a first-round match held over from Tuesday because of rain.

In the men's exhibition tournament at Kooyong, former home of the Australian Open, Tomas Berdych, the No. 7-ranked Czech, beat Spain's Fernando Verdasco 7-5, 6-2.

The Australian Open begins on Monday at Melbourne Park.