Juan Martin del Potro won the Sydney International last year, but saw his defence ended by Mikhail Kukushkin in the semi-finals on Thursday.
Defending champion Juan Martin del Potro's Sydney International title defence came to an end on Thursday as he suffered a straight-sets defeat to Mikhail Kukushkin in the quarter-finals.
The Argentine had entered the ATP 250 event as a wildcard, having spent almost a year on the sidelines with a problematic wrist injury.
Victory over top seed Fabio Fognini to reach the last eight hinted that Del Potro might be able to go all the way in his first tournament back, but qualifier Kukushkin claimed a 7-6 (7-5) 7-6 (7-3) victory in a match dominated by serve.
Neither player dropped a service game, or faced a break point for that matter, with 16 of 24 games being won to a single point or less.
Kukushkin found himself a break down in the first-set tie-breaker before bouncing back to edge the win, but won the second with room to spare.
The Kazakh will now face Leonardo Mayer - the last remaining seed in the competition - for a place in the final.
Mayer and Julien Benneteau had been the only two members of the tournament's top eight left in the draw following Wednesday's mass second-round exodus, and the Argentine fifth seed got the better of his French opponent to win 6-3 7-6 (7-4) and book his place in the last four.
A single break of serve proved enough to claim the opening set, and Mayer successfully fended off a break point early in the second before prevailing in the tie-breaker.
Gilles Muller eventually came out on top in a marathon second-set tie-breaker to progress past home favourite Bernard Tomic 7-6 (7-3) 7-6 (15-13).
Neither player dropped serve for the duration of the high-quality encounter, but Muller eventually converted his fifth match point to end the encounter in two sets.
Muller will now meet Viktor Troicki - a 6-3 3-6 7-5 victor over Simone Bolelli - in the semi-finals.