With this year's Shanghai leg of the newly launched IAAF Diamond League circuit a little more than a month away, the coach of Chinese hurdler Liu Xiang said the superstar will not be able to best 13 seconds in his latest comeback.

"It's impossible (for him to run within 13 seconds right now). We firstly have to make sure his physical condition is sound," said Sun Haiping during a Monday press conference to promote the Shanghai meet.

The meet is slated for May 23 and will be the season's first outdoor appearance for Liu. He made a successful return from injury last September at the IAAF Shanghai Golden GP with a stunning second-place finish in 13.15 seconds after a lengthy recovery from a career-threatening injury during the 2008 Beijing Games.

Since last September, he has experienced a number of minor setbacks to his right Achilles' tendon but has raced at four other meets.

Liu, who won the 2007 world championships and 2004 Athens Olympics, has yet to reach his best form.

"I am very excited that so many world class athletes will come to China and my hometown Shanghai to participate in the IAAF Diamond League," said Liu."We will do our best to give outstanding performances for local and global track and field fans."

The outing will also be the first of the 14-meet Diamond League for Jamaican star Usain Bolt, the world and Olympic 100m and 200m champion. Bolt is also the world record holder in each event (9.58sec and 19.19sec, respectively).

Usain or Liu could record great times at Shanghai, where the track is considered a fast surface. Last year's winning times in the men's and women's 100m sprints were 9.69sec and 10.64sec.

Sun said Liu's recovery should progress after the weather gets warmer. "I think it will be much better after the weather gets hot," he said.

The Diamond League circuit gets under way on May 14 in Doha and culminates in Brussels on Aug 27.