Former champion Liu Xiang left the Olympics Games in the first heat of the 110-meter hurdles for the second time in a row, clutching his right Achilles after he crashed into the first obstacle.

It left one of the signature races of the games without one of its main characters, as many had hoped to see China's Liu challenge defending champion Dayron Robles and this year's top performer Aries Merrit in the final.

The duel between 100 champion Usain Bolt and training partner Yohan Blake continued apace, with both cruising into Wednesday's semifinal of the 200, where Bolt is seeking his second Olympic sprint double.

Liu's hopes for a second gold are over – for at least another four years.

Liu won in 2004 at Athens to become the first Chinese man to claim an Olympic track and field gold medal, but shocked the home crowd in Beijing four years later when he had to withdraw in his heat without clearing a hurdle because of a right foot problem which later required surgery.

“He's one of the best,” Bolt said of Liu. “For him to push himself and come back last year and for this to happen, it's really sad for any athlete.”

This time, his coach had already expressed concern that Liu was less than fully fit, but still he had been expected to challenge for gold again in London.

“It was just terrible for that to happen to one of the best hurdlers of all time. It was just a tragedy,” American favorite Aries Merritt said.

Liu had his right Achilles protected with yellow tape when he came into the stadium. While other runners flew over hurdles for practice, he jogged up to the first hurdle and raised his eyebrows.

He produced a wan smile when the stadium speakers announced his name to the cheering. He was talking to himself as he started to settle in the blocks.

Then from the starting gun, it all went wrong for one of the biggest stars in the sport. With his lead leg, he crashed into the first hurdle and tumbled over. He looked up and held his right leg, realizing history has just repeated itself.

Instead of being wheeled off, he decided to hop on one leg along the track and went over to kiss the last hurdle in his lane 4. When he came to the finish line, Balazs Baji of Hungary was awaiting him and raised the arm of the former Olympic champion. (AP)