Liu Xiang (C) of China clears a hurdle next to Willi Mathiszik (R) of Germany and Ryan Brathwaite (L) of Barbados during their men's 110 metres hurdles heats at the IAAF World Championships in Daegu, Aug 28, 2011. [Photo/Agencies]

DAEGU, South Korea - China's Liu Xiang looked back to his best after three years of injury misery as he cruised through the opening round of the 110 meters hurdles on Sunday.

The 27-year-old former world and Olympic champion remained on collision course with American David Oliver and reigning Olympic gold medallist Dayron Robles, who both also eased into the semi-finals in Daegu.

Liu, who has raced infrequently and often in pain since his dramatic departure at the 2008 Beijing Olympics, looked comfortable with his newly adopted seven-step start and won the opening heat in 13.20 seconds.

"It was just the qualification so it was okay," the 2007 world champion and former record holder told reporters.

"I'm relaxed and surprised at the results. I think it will take 13 seconds or better to win."

The powerful Oliver, unbeaten in 18 successive finals until Liu bettered him in Shanghai earlier this year, was equally impressive as he barely grazed a hurdle in winning his heat in 13.27 seconds.

Oliver, who has run the only sub-13 second time (12.94) this year, said he did not put much stock in performances at this stage of the championships but declared himself pleased with his race.

"Race went well," he said. "Just all about running a time to get to the next round. I did that. I ran pretty easy today, really didn't try to press or do anything. I think I ran pretty good.

"As long as I keep doing that and run a little faster, I'll be straight."

David Oliver of the US clears a hurdle during their men's 110 metres hurdles heats at the IAAF World Championships in Daegu Aug 28, 2011. [Photo/Agencies]

Robles, who holds the world record of 12.87 seconds but has also struggled with injury over the last couple of years, was the least assured of the three fastest hurdles of all time.

The 24-year-old still had little trouble in reaching the semi-finals, however, easing up to finish in 13.42 seconds behind American Aries Merritt in heat four.

"I'm feeling good. I felt normal today. Everything is okay. Right now, I feel confident," he said.

The fastest time of the round came from Oliver's compatriot Jason Richardson, who confirmed his good form this year with a run of 13.19 seconds to win the second heat.

Defending champion Ryan Brathwaite of Barbados has struggled for fitness and form since winning the title in Berlin two years ago and tumbled out of the competition after running 13.57 and finishing fifth behind Liu.

Oliver would have been a strong favourite for the title a year ago but with the resurgence of Liu and Robles, the 29-year-old knows he is going to have to earn his first major global title.

"You definitely want to get something tangible back to your house so you can look at it, man," he added.

"(But) I'm already living a dream. I'm not even supposed to be doing this. I'm supposed to be circulating that resume by now. I didn't go to school to be no track star, didn't think about being a track star. It's all a big dream for me."