Former Olympic and world champion hurdler Liu Xiang has withdrawn from the World Challenge meet in Zagreb, Croatia, in September after failing to recover from his Achilles injury.

Although it has been more than a year-and-a-half since Liu underwent surgery in the United States, the inflammation around the injury has yet to heal completely, his coach Sun Haiping told the Oriental Morning Post Tuesday, adding that the hurdler was unable to undertake specialized training.

The 27-year-old Shanghai native is currently training in Beijing. He is also in line to compete at the Guangzhou Asian Games in November.

"Unless he is free of pain and in perfect condition, competing in all these events is meaningless," Sun told reporters in Beijing.

"If he doesn't do well, it will affect his confidence even further."

Liu flew to the US last month to have his foot checked and undergo some rehabilitation training. The result appeared good, Sun said.

"The surgery was a success but the inflammation existed even before the surgery," Sun said. "Because of the bursitis, we can't force any specialized regime on Liu."

Sun said rehabilitation and recovery was Liu's immediate priority ahead of the 16th Asian Games.