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Sydney - Juan Martin del Potro's winning comeback after 10 months out through injury was ended by qualifier Mikhail Kukushkin at the Sydney International on Thursday.
The Argentine defending champion, who spent much of last year recovering from wrist surgery, was knocked out in two tiebreaker sets by the Kazakhstan qualifier, 7-6 (7/5), 7-6 (7/3) in just under two hours.
Del Potro, who beat Sergiy Stakhovsky and top seed Fabio Fognini to reach the quarter-finals after he was awarded a wildcard into the tournament, fought to the end, but found Kukushkin too strong on the day.
Kukushkin will now play another Argentine fifth seed Leonardo Mayer, who got past French fourth seed Julien Benneteau, 6-3, 7-6 (7/4) in Friday's semi-finals.
Luxembourg's big-serving Gilles Muller will play Serbia's Viktor Troicki in the other semi-final.
Muller needed five match points in an epic 28-point tiebreaker before prevailing over Australia's Bernard Tomic, 7-6 (7/3) 7-6 (15/13) on Ken Rosewall Arena.
It was the fourth time in as many meetings that Muller has beaten Tomic.
Troicki had a hard-fought 6-3, 3-6, 7-5 win over Italy's Simone Bolelli in a match that lasted over two hours.
The world No.92 saved a match point in the deciding set then converted one of four break points to seize momentum of the match and close out the victory.
Only a handful of tiebreak points separated the Kukushkin and del Potro with neither player's serve broken during the match.
Del Potro, the 2009 US Open winner, ran out of steam against the world No.66, who made it six straight wins in the tournament after three victories in qualifying.
"It was a tough match," he said.
"I think he deserve to win because he played better than me in the tiebreak.
"The match was close. I served well, but my wrist hurts a little bit more than yesterday, and I couldn't hit harder than my last match."
Del Potro was satisfied with his week's work and some important match practice ahead of next week's Australian Open in Melbourne.
"It's a great week for me. I did more than what I expected before," the former US Open champion said.
"I played three matches in a row and I played against good players in this tournament, which means a good comeback for me.
"I'm happy just to be playing tennis again against these players. It will be more interesting for me and I will be improving faster than training at home with my coach."
It was a significant win for Kukushkin, who looms as a potential roadblock for a seeded player at the Australian Open.
Kukushkin said he could see that del Potro was below his full power and struggled on his backhand.
"He's a top player, but I think everyone saw today that he cannot yet hit full power (on) his backhand," he said.
"He did a lot of slices. For sure this affected his game. Still he serves good. His forehand was working okay today."
Collated results from the fifth day of the Sydney International tennis tournament on Thursday (x denotes seed):
Quarter-finals
Mikhail Kukushkin (KAZ) bt Juan Martin del Potro (ARG) 7-6 (7/5), 7-6 (7/3)
Leonardo Mayer (ARG x5) bt Julien Benneteau (FRA x4) 6-3, 7-6 (7/4)
Viktor Troicki (SRB) bt Simone Bolelli (ITA) 6-3, 3-6, 7-5
Gilles Muller (LUX) bt Bernard Tomic (AUS) 7-6 (7/3), 7-6 (15/13)
