Del Potro got by Radek Stepanek 6-4, 3-6, 6-3. He had 14 aces and fought off seven of Stepanek's 10 break chances. Del Potro cashed in both of his break opportunities in the third set while dropping just seven points on serve.
Bernard Tomic, the tournament defending champion, continued his run of strong play in Sydney with a 6-4, 6-3 decision over Alexandr Dolgopolov, whom Tomic had beaten just once in six previous meetings. Thursday, however, Tomic piled up three breaks in the first set and had the only break of the second to gain the semifinals.
Fourth-seeded Dmitry Tursunov topped Denis Istomin 7-6 (7-5), 6-2 by winning two-thirds of the points on serve and saving all three of the break points he faced. Qualifier Sergiy Stakhovsky moved into the semifinals with a 6-4, 6-3 win over Marinko Matosevic.
Del Potro and Tursunov square off in the first semifinal Friday. Del Potro is 2-0 versus Tursunov although their last match, last year in Cincinnati, went three sets. While both those players are ranking in the Top 40, Tomic is 52nd and Stakhovsky is 101st in the rankings. Tomic beat Stakhovsky in a 2012 match in their only career meeting.