Del Potro crashes in Sydney

The Argentine defending champion was beaten by the Kazakhstan qualifier 7-6 (7-5) 7-6 (7-3).

Neither player's serve was broken during the match but Kukushkin looked the sharper of the two as the match progressed.

The 2009 US Open winner seemed to run steam against Kukushkin, this after being out of action for month's recovering from wrist surgery.

Kukushkin next faces Argentine Leonardo Mayer in the semi-finals, who beat France's Julien Benneteau 6-3, 7-6 on Thursday.