Jamaican star sprinter Usain Bolt lauded Chinese hurdler Liu Xiang a "true champion" despite his heartbreaking elimination from the men's 110m hurdles heat on Tuesday.

"It must be hard for that to happen for the second time in a row. He's a great athlete," said Bolt, who successfully retained his Olympic crown in men's 100m dash by clocking 9.63 seconds Sunday night.

"One of the most amazing things is that he came back. He switched legs and started leading with a different leg. He's a true champion. It's so bad for him," he added.

Liu Xiang, 29, exited the London Games after smashing into the first obstacle of his 110m hurdles heat, four years after he was forced to withdraw due to a tendon injury from the event at the 2008 Beijing Games.

Chinese athletics team leader, Feng Shuyong later revealed that Liu might have snapped his Achilles.

"I think the problem happened with the taking off. We all know that at the moment of taking off the tendon would have very, very big pressure," said Feng.

Liu earned a rousing ovation from the 80,000 crowd as he hopped down the track to symbolically cross the finishing the line, kissing the last hurdle as he did so before being consoled by fellow competitors.