Doha - A mystery injury scuppered Michael Norman's bid for 400 metres gold on Wednesday as the up-and-coming American star crashed out in the semi-finals at the Khalifa Stadium.
The 21-year-old USA sprinter, the fastest man in the world this year over the distance, had been one of the favourites for gold in Doha alongside compatriot Fred Kerley.
However Norman was always struggling in a heat won by Trinidad's Machel Cedenio, finishing last in seventh place in a time of 45.94.
Afterwards, Norman revealed he had been troubled by a niggling injury in the "lower half" of his body for the past two months.
He declined to elaborate on the precise nature of the injury.
"I came into the race with the mentality of going out there," Norman. "But my body gave me warning signs today and I listened to them and made the executive choice to slow down instead of risking injury.
"It's real disappointing to come out here after 330 days of training and not even make the final, not even look good in the semi."
Norman said he had been managing the problem since it first flared up with the help of his trainer.
"We've been taking it day by day, step by step, breath by breath, moment by moment," he said.
"I've always approached each new challenge as it presented itself. Today my body got the best of me.
"It's disappointing but I just have to take this performance for what it is, and be a better athlete for next year."
With Norman injured, the USA's hopes of completing a 100m, 200m, 400m and 800m sweep now rest with Kerley, who won his heat in 44.25sec.
Kerley can face a strong challenge from the Bahamas' Steven Gardiner, who won his heat in 44.13sec, the quickest time of the semi-finals.
Grenada's Kirani James, the 2012 Olympic champion, qualified for Friday's final with the second quickest time of 44.23sec.