Dai Greene told BBC Sport "I just didn't have it in the tank", after failing to qualify for the final, to defend his 400m hurdles world title.
The 27-year-old from Llanelli has been struggling with fitness and illness going into the World Championships at Moscow.
Bridgend athlete Rhys Williams also failed to make the 400m hurdles final.
Welsh athletes Dai Greene and Rhys Williams have both failed to qualify for the final of the 400m hurdles at the World Athletics Championships in Moscow.
Greene, the defending champion from 2011, has been dogged by injury and illness and finished his semi-final in 49.25 seconds, down in fifth place.
Williams was fourth in his semi-final, in a time of 49.29 seconds - not good enough to make the final.
Welsh athletes Dai Greene and Rhys Williams will compete in the 400m hurdles semi-finals at the World Athletics Championships in Moscow this afternoon.
Greene was ill over the weekend, adding to a calf tear that has hampered his build-up. "Fingers crossed I can rest up enough and put in a better performance," he said. "If I feel like this [for the semi-final] then realistically I'll have no chance of getting a medal, but I hope I'll feel better."
Dai Greene was the 15th-fastest qualifier overall for the 400m hurdles semi-finals at the World Athletics Championships.
The defending world champion, who finished fourth at London 2012, 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."
"We were wondering if I'd be good enough to run today. We didn't know what I was capable of. Hopefully I'll be a bit better tomorrow. If I feel like this tomorrow then realistically I'll have no chance of getting a medal, but I hope I'll feel better."
Brett Morse has failed to qualify for the discus final at the World Athletics Championships in Moscow.
The 24-year-old from Penarth could only 59.23 metres to finish 11th in his qualification group this morning.
Rhys Williams will join Dai Greene in the 400m hurdles semi-finals at the World Athletics Championships.
The 29-year-old from Bridgend finished fourth in his heat this morning to progress.
Welsh hurdler Dai Greene struggled through his heat - as he began the defence of his world 400m hurdles title in Moscow.
The 27-year-old from Llanelli came home fifth in the final heat, which would have left him relying on a fastest loser spot, but moved up to fourth when Puerto Rico's Eric Alejandro was disqualified.
That secured him a place in tomorrow's semi-finals.
Greene won gold in the 400m hurdles in Daegu in 2011.
Welsh hurdler Dai Greene will begin the defence of his 400 metres hurdles crown at the Athletics World Championships in Moscow later.
The 27-year-old won the world title in Daegu two years ago.
Fellow Welsh athlete Rhys Williams is also set to run in the heats.
Welsh athlete Rhys Williams has come second in the 400m hurdles at the Birmingham Grand Prix this afternoon. He finished strongly in a time of 48.93 seconds.
However, there was disappointment in the same race for Llanelli's Dai Greene who finished fifth in a field of eight competitors. Greene has recently been recovering from a double hernia operation.