BRISBANE - US Open champion Andy Murray won a surprisingly tough second-round clash against Australian qualifier John Millman to move into the quarter-finals of the Brisbane International on Thursday.

Murray appeared headed for a straightforward win when he raced through the first set in just 30 minutes, only for an inspired Millman to fight back and deservedly claim the second. But the world number three regrouped and took control at 2-2 in the decider to ease away and win 6-1, 5-7, 6-3 in just over two hours.

Millman, 23, is a Brisbane local who spent most of 2012 on the Challenger and Futures circuits, entering the Brisbane International ranked 199 in the world. His match against Murray was only his fifth on the main ATP tour.

However, he never looked overawed against the world number three and played some spectacular tennis to give Murray a huge scare. "I'd not seen him play before but I knew he has a good work rate," Murray said. "He's gone through a lot of tough losses and kept fighting and he had an injury a few years ago and came back. He's a lot better than his ranking, that's for sure. I expected a tough match and didn't expect him to go away, and he didn't."

Murray was clutching his stomach and legs at various stages throughout the second and third sets, but stressed he had no injury concerns. "It's incredibly humid here and I've had a lot of travelling too, and there were so many long points of very high intensity," he said. "It's just taking a little while to adjust to playing matches again after such a long period. At the beginning of most years the first few matches can be tough on the body, but you get used to it and you feel better and better."

 That was the perfect first match for me in many ways, to go through some tough moments, a lot of long rallies and close games and important points, and to come through."

Murray now plays Uzbekistan's Denis Istomin in the quarter-finals after Istomin downed veteran Australian Lleyton Hewitt 7-5, 7-5. Earlier, rising Bulgarian star Grigor Dimitrov upset second seeded Canadian Milos Raonic 6-3, 6-4. Dimitrov served marginally better than Raonic and took his chances when they came, breaking the Canadian once in each set to wrap up victory in just over an hour. He will now play Jurgen Melzer in the quarter-finals after the seventh seeded Austrian beat Belgian David Goffin 6-4, 7-6 (7/4).