Isner, who was knocked out at the quarter-final stage last year, overpowered the 16th ranked Robredo with 20 aces to run out a 7-6 3-6 6-4 winner.
Big-serving American John Isner advanced to the Auckland Open semi-finals for the first time with a three-set victory over top seed Tommy Robredo on Thursday.
The 24-year-old Isner, who was knocked out at the quarter-final stage last year, overpowered the 16th ranked Robredo with 20 aces to run out a 7-6 3-6 6-4 winner.
"It was tough... and I certainly had to dig deep," the popular Isner told reporters of his third three-set victory this week. "Down a break in the first. Down a break in the third. I had to play pretty well to keep my head and that's what I did."
A noticably frustrated Robredo said that while he had broken Isner's booming serve three times, he still had trouble with a ball that was clocked moving at more than 220km per hour (137mph).
Robredo, who helped Spain to Hopman Cup victory last week, said he was nevertheless pleased with his form heading into next week''s Australian Open.
"I feel good. I'm not going to change much from the way I played or am feeling," Robredo said.
"I played great from the baseline, was returning great and only had a couple of mistakes with my serve. The rest is positive for me, even though I lost, but if you play a guy like John, it can happen."
Robredo's demise ensured that none of the top four seeds would be in the semi-finals on Friday.
Isner will now meet Spain's Albert Montanes in the semi-finals after the eighth seed beat Swiss qualifier Michael Lammer 6-3 6-3.
By making light work of the 209th-ranked Lammer, Montanes has advanced to the semi-finals without having to beat anybody ranked in the top 200 in the world.
Isner said he doubted if the ease of the Spaniard's path would have any bearing on their match.
"He has a deceptively strong serve," Isner, who beat Montanes in Auckland last year, added. "He's also very fast and has some great groundstrokes... so it will be tough."
France's Arnaud Clement continued a strong run with a 6-2 7-5 victory over sixth-seeded Austrian Juergen Melzer and will meet 2008 champion Philipp Kohlschreiber, who beat France''s Marc Gicquel 6-3 6-1 in the late match, in the final four.
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