A foot injury may force defending Olympic decathlon champion Dan O'Brien to miss the Sydney Games.

O'Brien, 34, said he injured his foot while practising the high jump last week and doesn't think he'll be healthy enough to participate in the U.S. trials, which begin on Thursday.

"I don't see any way I can participate," O'Brien said at a news conference. "It's heartbreaking. I'm in shock."

If O'Brien misses the trials he's ineligible for the Olympics in September.

O'Brien said he's tried everything from pain-killers to tightly wrapping the foot but nothing seems to help.

"It hurts too much even to jog at this point," said O'Brien. "I can't even take three or four steps without having to stop. It's that painful."

O'Brien said he's "98 to 99 per cent" sure that he won't participate in the trails, but won't actually withdraw until right before the competition begins.

"If for some reason a miracle were to happen, my prayers were answered and I woke up Thursday morning and felt like I could sprint, I would be out there giving a try," O'Brien said.