Sydney, Australia (SportsNetwork.com) - Defending champion Juan Martin del Potro came up a quarterfinal loser Thursday at the Sydney International tennis event, a final Australian Open tune-up.

Kazakhstani qualifier Mikhail Kukushkin doused his wild card fellow non-seed del Potro in 7-6 (7-5), 7-6 (7-3) fashion on the hardcourts at NSW Tennis Centre.

The oft-injured 2009 U.S. Open champion del Potro had won his first two matches this week after being sidelined since last February with a wrist injury.

The tall Argentine slugger beat Aussie and 2013 Sydney titlist Bernard Tomic in last year's finale here.

Tomic also lost on Wednesday, as Luxembourg's Gilles Muller snuck past the inconsistent Aussie, 7-6 (7-3), 7-6 (15-13).

Meanwhile, fifth-seeded Argentine Leonardo Mayer upended fourth-seeded Frenchman and 2012 runner-up Julien Benneteau 6-3, 7-6 (7-4) and 2011 Sydney runner-up Viktor Troicki of Serbia outlasted Italian Simone Bolelli 6-3, 3-6, 7-5.

Friday's semifinals will pit Mayer against Kukushkin and Muller versus the qualifier Troicki.

This week's winner will claim $80,000.