Dai Greene, the current world champion in the 400m hurdles, failed to make an impact in the Olympic final on Monday night, finishing fifth.
The runner, who has been dogged by injuries in the build up to the Games, got off to a slow start in the race and, despite a push in the final 100m, was never close to getting on the podium.
Greene finished in 48.24 seconds.
The race was won by Felix Sanchez, 34, from the Dominican Republic, who took the gold eight years ago in Atlanta.
Greene, 26, had struggled in the semi-final, qualifying as one of the fastest losers.
Speaking to the BBC after the race, the Welshman said: "I felt a bit tired tonight. Obviously it was a big shock in the semi-final - I was surprised everyone went that fast.
"I gave it everything I had tonight. There was a better structure to my race but I was just a bit too tired at the end there and narrowly missed out."
"I just think perhaps the endurance wasn't quite there for the rounds which shows I missed some of the winter work. You can't always tell if it's there or not until you get into these majors.
"I was just found wanting a bit at the end but I couldn't have given anything else tonight."
During the medal ceremony, Sanchez struggled to contain his emotion, weeping throughout the presentation. He received huge applause from the stadium crowd at the end of the Dominican Republic national anthem.