World champion Sun Yang of Zhejiang swims en route to 800m freestyle title at the national championships in Shaoxing, Zhejiang Province.

Former world champ fails to qualify for Olympics, and time is running out.

It's not over yet, but the chance is slipping away.

Missing a shot at an automatic berth for the London Olympics due to his lackluster performance at the national trials, China's first men's world champion swimmer Zhang Lin was shattered but remains upbeat about the future.

"I am really tired and the results here are so disappointing," an emotional Zhang said after finishing sixth in the1,500m freestyle on Sunday. "But I won't give up as long as I still hold a slice of a chance."

Zhang was dealt a blow last Tuesday when he finished fourth in his favorite event, the 400m freestyle, with a time far off the Olympic A standard.

Disappointment hit again when Zhang failed to snatch Olympic spots later in the 200m and 1,500m, finishing 10th and sixth respectively.

Entering the event revitalized, Zhang had expected it to be a confidence boost. His career has fallen off since the 2010 Asian Games.

Instead, it turned out to be just another bad week.

"I don't know what's wrong with my performance. It was supposed to be much better - I've regained my rhythm and done tests well in training," Zhang said after clocking 3 min, 49.14 sec in the 400m, 6.83 sec slower than winner Sun Yang.

After winning the 400m freestyle silver at the 2008 Beijing Olympics and the 800m freestyle gold at the 2009 Rome World Championships, Zhang emerged as the leader of the men's team.

But tough training and competition schedules in recent years exhausted him, resulting in deep fatigue and a lack of motivation.

The 25-year-old has seen just how unmerciful competitive sports can be.

"I've been adjusting my mentality," he said. "One race shouldn't erase all the effort I've put in. That's the cruel part of sports. Winning or losing happens every day. As an athlete, I'm prepared for it."

Coach Chen Yinghong said Zhang's sub-par performance was probably due to an asthma attack.

"I was struck by the result. I think the asthma he showed before the game was the only explanation," said Chen, who's been coaching Zhang for 10 years.

Although players' performance in Shaoxing is a key component to the selection process, the governing body will also take prior results at major events into account as it decides the final Olympic lineup, said Shang Xiutang, vice-director of the Chinese Swimming Association.

Zhang has to reach the mandatory Olympic A standard while clocking a national top-two result at an event to clinch a spot on the final roster before the July 3 deadline.

The next domestic qualifier is the national youth championships in June. Results from international events will also be recognized if Zhang reaches the standard.

"We will go back and continue training for the next available event," Chen said after the 1,500m.

Teammates and swimming celebrities offered encouragement.

"I've experienced the same kind of tough period, so I understand the frustration," said retired female swimmer Luo Xuejuan, who won the100m breaststroke gold medal at the 2004 Athens Olympics. "He doesn't need to prove anything. Just tune up and take it easy. Wish him all the best."

Retired butterfly swimmer Zhou Yafei also reached out to Zhang on his micro blog, saying "Never give up! You will see lights at the end of darkness", while Zhang's national teammate Wu Peng wrote "Believe in your own faith".

Meanwhile, the men's team's new leader, Sun, who is the nation's top hope for gold in London, also paid tribute to Zhang by raising his hand in the air after they finished the 1,500m free in adjacent tracks.

"He has contributed so much for Chinese swimming. It's tough for him to continue fighting. He deserves everybody's respect," said Sun, the 1,500m world record holder.