Roddick, bidding for a third title in five years at the Midwest venue, went down in a hail of 15 aces from the 109th-ranked player in the world, who had never played even a quarterfinal on the ATP before this breakthrough week.

James Blake, the second-seeded defending champion who beat Roddick in the 2006 final, was bundled out by another big hitter in a rain-delayed quarterfinal.

Teenaged Californian Sam Querrey fired 34 aces in a 7-6 (10/8), 6-7 (4/7), 7-6 (7/4) over Blake.

Dancevic, travelling without a coach but with the moral support of his girlfriend, will face off Sunday against Russian third seed Dmitry Tursunov.

Tursunov beat Japanese amateur Kei Nishikori 6-1, 6-4 in another rain-delayed quarterfinal then came back to beat Querrey 7-6 (7/5), 6-2.

The Russian converted his fourth match point to book his final berth.

Tursunov defeated Dancevic two months ago in the second round of the grasscourt Queen's Club tournament in London.

Dancevic bombed four aces midway through the first set against Roddick to level at 4-all, then broke the American after saving a pair of break points himself.

The 22-year-old underdog took the set and then broke to start the second, giving the break back in the fourth game.

But Dancevic kept his composure on the big occasion against the former world number one, the Canadian ending his last three service games with aces to force a tiebreaker.

He dominated in the decider to earn the upset in just over 90 minutes.

Roddick was disgusted with another easy loss just a month after going out in a Wimbledon semi-final from a position of strength against Richard Gasquet.

"I didn't have much to give out there today, I don't know why. I wasn't feeling that well," Roddick said. ""I served badly, but I've been doing that all week. I don't know what happened."

Dancevic, the last man into the draw by ranking, didn't have a clear idea either of how he had achieved his miracle.

"I haven't thought about being in the final, I'm just enjoying the moment," he said. "My only game plan was not to get broken, I didn't want to get behind against him.

"It seemed that when I got stressed out, I would hit a few good shots and win the game. That happened two or three times."

Dancevic added: "I'm happy that I got through today, this is one of the best matches of my career."