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Sydney, Australia (SportsNetwork.com) - Top seed Juan Martin del Potro was a hard-fought quarterfinal winner Thursday at the Sydney International tennis event.

The menacing world No. 5 Argentine held off Czech veteran Radek Stepanek 6-4, 3-6, 6-3 in 2 hours, 8 minutes on Day 4 at NSW tennis Centre.

The former U.S. Open champion del Potro's semifinal opponent on Friday will be fourth-seeded Russian Dmitry Tursunov, who bested Uzbekistan's Denis Istomin 7-6 (7-5), 6-2 at this final hardcourt Australian Open tune-up. Tursunov was the Sydney champ in 2008.

Friday's other semi will pit Ukrainian qualifier Sergiy Stakhovsky against reigning champion Bernard Tomic of Australia. Stakhovsky stifled Aussie wild card Marinko Matosevic 6-4, 6-3, while Tomic handled Ukrainian Alexandr Dolgopolov 6-4, 6-3.

Tomic topped South African Kevin Anderson in the final to capture his first ATP World Tour title in Sydney last year.