China's Liu Xiang reacts after competing the heats of men's 110m hurdle during the star-studded Aviva London Grand Prix at the Crystal Palace in London, July 13, 2012. Liu pulled out of Diamond League London GP for back injury. [Xinhua/Wang Lili]

China's star hurdler Liu Xiang's back injury which forced him out of the London Diamond League finals is "not serious", his long-time coach Sun Haiping said in London on Friday.

"Liu Xiang felt a little uncomfortable in his back after the heat round," Sun said at south London's Crystal Palace stadium. " We decided to pull out of the finals just for the sake of caution. "

It's the first time for the Chinese hurdler to retire during the competition after his shock withdrawal from the Beijing Olympic Games semifinals in the Bird's Nest Stadium citing an Achilles injury.

Liu, running in his last competition here on his 29th birday, has been pinned high hopes as four years ago by Chinese fans to win a second gold medal following his victory in Athens 2004.

"I am still under a lot of pressure although I am away from home this time," he said on Thursday. "There are too much expectations from the fans."

The London Grand Prix should have been a perfect chance for him to warm up for the Olympics. Besides the weather, he could race against American duo Aries Merritt and Jason Richardson, two of the biggest threats in the Olympic Games.

Liu has maintained good form this season, beating Merritt and Richardson in Shanghai and Eugene Diamond League and registering a wind-assisted 12.87 seconds in Eugene, Oregon.

He had cruised into the 110m hurdles final after winning his heat in 13.27 seconds but failed to take his place on the start line for the final for the injury.

Merritt defied the cool, overcast conditions to equal his own season's fastest time of 12.93 seconds. Richardson was second in 13.06 and another American Ryan Wilson third in 13.18.

Liu will remain in London to prepare for the Olympic Games. The men's 110m hurdles title will be decided on August 8 at the Olympic Stadium.