Juan Martin del Potro advances

SYDNEY -- Former US Open champions Juan Martin del Potro of Argentina and Samantha Stosur of Australia rallied to win their second-round matches against tough French opponents Wednesday at Australian Open warm-up tournaments.

Del Potro beat Nicolas Mahut 1-6, 6-3, 6-4 at the Sydney International, while Stosur saved two match points and eked out a 6-4, 2-6, 7-6 (7) win over Kristina Mladenovic at the Hobart International.

Stosur lost all three of her singles matches at the Hopman Cup team exhibition event last week and is looking for some confidence heading into her home Grand Slam at Melbourne Park, where she has never advanced past the fourth round.

"Any win's a good win, but when you've got to really tough it out and battle through a long match ... you're probably holding your nerve pretty well," Stosur said.

The Australian Open begins Monday.

Earlier, former Wimbledon champion Petra Kvitova advanced to the semifinals of the women's event in Sydney with a 7-6 (4), 6-2 victory over Lucie Safarova. Kvitova, the second seed, will play Tsvetana Pironkova in the semifinals after the Bulgarian's 7-6 (2), 6-3 win over Sara Errani.

In other men's matches at Sydney, defending champion Bernard Tomic of Australia reached the quarterfinals with a 6-3, 4-6, 6-4 win over Blaz Kavcic of Slovenia, as did his compatriot Marinko Matosevic, who upset third-seeded Andreas Seppi of Italy 6-3, 6-4.

In Hobart, defending champion Elena Vesnina retired from her second-round match with a back injury. Spanish qualifier Estrella Cabeza Candela was leading the match 4-6, 6-2, 4-1 when Vesnina quit.

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 Haas 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 Bautista Agut in Thursday's quarterfinals after the Spaniard beat sixth-seeded Benoit Paire of France 6-3, 6-4.

David Ferrer, the No. 1 seed and defending champion, beat Donald Young of the United States 6-7 (5), 6-3, 6-1. The Spaniard is seeking his fifth Auckland singles title.

Third-seeded American John Isner also progressed to the quarterfinals with a 7-6 (1), 3-6, 6-2 win over Lukas 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, Tomas Berdych of the Czech Republic beat Spain's Fernando Verdasco 7-5, 6-2, and Switzerland's Stanislas Wawrinka defeated Frenchman Gilles Simon 6-4, 5-7, 6-3.

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