Dai Greene, Team GB captain and world hurdles 400m champion, had the scare of his life after inexplicably failing to finish in the automatic qualifying places in his semi-final.
Greene appeared broken at the end of his race as his lay flat on his back and covered his face with his hands.
He finally qualified in ninth place with a time of 48.19secs, but he says he's "gutted" with the result.
Gutted: Greene says he was gutted with the result
"I came off the bend too far down and there was too much to do. I can't really explain how I feel. I'm just gutted.
"I don't think I went off too hard. I should have been better than that and I'm very disappointed.
"It's no way to perform if you are world champion. Before I knew it they were up on me and as soon as they were up on me, they were past me.
"I had nothing left to give at the end. I should be able to produce better than that. I know I'm better than that. I'm very disappointed in myself."
The final takes place on Monday night at 8.45pm.