Unseeded Croat Ivo Karlovic sent top seed Lleyton Hewitt crashing out of Queen's with a swift 7-6 6-3 victory.
Booming serves were the story of the match and neither player could carve out a break-point in a tight first set.
But it was the 6ft 10in Karlovic who went ahead in the tie-break, clinching it 7-5 with two powerful serves.
Hewitt could find no way past the Croat, who broke Hewitt at 5-3 in the second before calmly serving out for the match and a place in the last four.
It was not the first time Karlovic has handed Hewitt, a three-time Queen's champion, a shock defeat.
The Croat ousted the Australian in the first round of Wimbledon in 2003, when Hewitt was attempting to defend his title.
Defending Queen's champion Andy Roddick, meanwhile, advanced to the semi-finals with an efficient 6-4 7-6 victory over Frenchman Sebastien Grosjean.
The first set was a solid affair before Grosjean inexplicably fell away in the 10th game, handing Roddick the set with a pair of double-faults.
Grosjean made amends - grabbing a break to go 2-0 up in the second but Roddick broke back to force the tie-break.
And the big-hitting American clinched the breaker 7-5 to wrap-up the match.
Fourth seed Radek Stepanek of the Czech Republic joined Roddick in the semi-finals after beating 10th seed Richard Gasquet of France 3-6 6-1 6-2.