SYDNEY (Reuters) - Champion Andy Roddick weathered an early onslaught from hard-hitting Ukrainian Alexandr Dolgopolov before beating him 6-4 6-1 to set up a quarter-final clash with Marcos Baghdatis at the Brisbane International on Wednesday.

The 22-year-old Dolgopolov was the early aggressor, attacking Roddick's serve to win an early break and go up 4-2.

The American world number eight quickly broke back, however, and reeled off 10 of the final 11 games to seal the match in less than an hour.

"I knew that he was aggressive to the point of psychosis," he said. "He came out in the first six games and I didn't really do much wrong."

"When someone is going to try and hit a winner from whatever ball they get on the very first shot, and if they convert, there's very little you can do.

"It was my plan to make him come up with those shots for the better part of a couple of hours and it caught up to him a little bit."

Roddick rocketed down 15 aces in the match to raise A$1,500 ($1,508) in donations for victims of devastating floods in northern Queensland state that have affected 200,000 people across an area larger than France and Germany combined.

"It's a very small part what we're doing here. It was Sam Stosur's idea," said the 2003 U.S. Open champion, who joined the Australian women's world number six in pledging A$100 per ace.

The second-seeded Roddick defeated Czech Radek Stepanek in the final last year on the way to a quarter-final appearance at the Australian Open.

Fifth seed Baghdatis thumped Poland's Lukasz Kubot 6-2 6-2 in just over an hour and was relishing the prospect of meeting Roddick on Thursday.

"It's nice to play guys like Andy," the former Australian Open finalist said.

"It's a test match before the Australian Open and it will be a great match... It's a good test tomorrow because it's Andy Roddick and he's going to push me to the limits."

Sixth seed Feliciano Lopez, who had withdrawn from doubles event with a virus, had his singles campaign cut short as well after his 6-3 6-1 defeat by unseeded South African Kevin Anderson.

In contrast, 2009 winner Stepanek, tamed fourth seed Mardy Fish 6-3 6-1.

In the women's draw, Jelena Dokic, battling a wrist injury and a stomach virus, could not make the most of her wildcard and received a 6-0 6-1 plastering from Germany's Andrea Petkovic before joining compatriots Stosur and Sally Peers on the sidelines.

(Reporting by Ian Ransom and Amlan Chakraborty; Editing by John O'Brien and Pritha Sarkar)