Dai Greene's lack of fitness was clear as he began the defence of his world 400 metres title by struggling through his heat in Moscow.
The Welshman admitted ahead of the championships that he had endured a "really depressing last few weeks", with a calf tear forcing him out of his last two races and severely hampering his training.
He accepted he would be stepping into the unknown in the Russian capital, and could only manage fourth as he booked his semi-final place on Monday morning.
He actually came home fifth, which would have left him relying on a fastest loser spot, but moved up a place when Puerto Rico's Eric Alejandro was disqualified.
The 27-year-old, who is far from the form which fired him to World Championship gold in Daegu two years ago, faded down the home straight to come home in 49.79 seconds.
Greene, fourth at London 2012, has endured a torrid season following a double hernia operation in March and a virus which led to him suffering his first defeat by a fellow Briton in five years
He was the 15th-fastest qualifier, American Olympic silver medallist Michael Tinsley heading the pile with 49.07secs.
And the Welsh athlete revealed that, to add to his injury worries, he had been ill over the weekend.
Greene said: "I've had a tough few weeks. I've not been able to do as much hurdling and it showed a bit at the end.
"I was ill on Friday and Saturday, so it's been a nightmare few days to be honest. I came down with some fever on Friday night and I've just been trying to eat and drink as much as I can to get my energy up.
"We were wondering if I'd be good enough to run today. We didn't know what I was capable of.
"We've got the answer and it's not the kind of answer we like. But hopefully I'll be a bit better tomorrow. I've had this situation before where you do one race and then feel a lot better the next day as a result of it. Fingers crossed I can rest up enough and put in a better performance tomorrow.
"The calf's been good. I hurdled on Friday for the first time, only over half a dozen hurdles, but that's all we wanted to do. We don't want to risk it, just to get the confidence it was working again.
"If I feel like this tomorrow then realistically I'll have no chance of getting a medal, but I hope I'll feel better.
All the Britons progressed from their heats, with European champion Rhys Williams also far from impressive in finishing fourth in 49.85s.
Sebastian Rodger, making his senior debut was the quickest of the trio in 49.66, even if his fifth-placed finish meant he went through as a fastest loser.