Top-seeded Juan Martin del Potro beat Czech veteran Radek Stepanek 6-4, 3-6, 6-3 on Thursday to set up a Sydney International semifinal against Russia's Dmitry Tursunov.
Del Potro, the 2009 U.S. Open champion who finished last season ranked No. 5, relied on his strong serve, with 14 aces to three, and saved seven of ten break points.
Tursunov had a 7-6 (5), 6-2 win over Denis Istomin of Uzbekistan. Defending champion Bernard Tomic was playing Alexandr Dolgopolov of Ukraine in a later quarterfinal.
At Hobart, top-seeded Sam Stosur reached the semifinals with a 6-3, 6-2 win over No. 34-ranked Bojana Jovanovski of Serbia.
The 2011 U.S. Open champion next faces seventh-seeded Klara Zakopalova of the Czech Republic, who beat American Alison Riske 6-4, 7-5.