NAIROBI, Kenya (AP) David Rudisha has been left behind, and he's not used to it.
After two years of injury setbacks, the 800-meter Olympic champion, once untouchable over two laps, has endured a slow, frustrating battle for fitness.
Meanwhile, his rivals - including Olympic silver medalist Nijel Amos - are up to speed ahead of the world championships in Beijing next month.
As he prepares for London's Diamond League meet this weekend, his return to the 2012 Olympic Stadium and the scene of his greatest triumph, Rudisha is focused instead on what might now be his biggest challenge: Just being 100 percent healthy again.
''It's very tough. I don't know how (else) I can say it,'' Rudisha told The Associated Press in an interview.
Since he first felt pain in his right knee in May 2013, Rudisha has had a succession of problems. The knee required surgery and rehabilitation in Germany. His return last year was then put on hold when he sustained a left calf injury while training at home. He was hampered again this year by a muscle problem in the right leg and had to withdraw from meets in the Czech Republic and England.
What it all means is Rudisha, who produced one of the finest examples of all-out attacking running to win the 800 at the London Olympics, obliterating the field and his own world record in the process, is unsure right now how far he can push himself.
''I better take it nice and easy, the best my body will allow me rather than go out there and forget I have a problem,'' the Kenyan said.
Rudisha said if he ignores his injury issues, ''Boom! Everything backfires. I don't want that.''
Now 26, Rudisha has had positive recent moments, winning at the New York Diamond League meet, though his 1 minute, 43.58 seconds was nearly three seconds slower than the world record he set in London.
He led for 600 meters in Lausanne, Switzerland, this month, but was chased down and beaten in the straight by Amos, the runner from Botswana who also beat Rudisha at last year's Commonwealth Games.
The latest loss to Amos will be another psychological blow for Rudisha, who three years ago ran Amos to the point of exhaustion. Amos had to leave on a stretcher after trying and failing to catch Rudisha in the Olympic final.
This loss to Amos, Rudisha said, was a learning experience, a strangely humble position from the owner of the world record, the Olympic title and the three fastest times ever in the 800. Rudisha said he was happy just to compete again as he tries to get some form ahead of his return to the worlds for the first time since 2011.
''I feel better than last year which is important,'' Rudisha said. ''I have learned my weakness and for the few weeks I have I will see how best I will work on that.''
The Diamond League race in London on Saturday may be a marker for Rudisha, if he can revive some of the memories of that devastating victory at the Olympics, the best 800 ever.
Rudisha thinks he'll get back to where he was at his peak.
''I've been having a lot of ups and downs, but I'm happy,'' he said. ''I thank my coach and my family who have been supporting me ... we have always been finding a way of fighting back and coming back.
''I always come (back) strong and do even better.''