Rhys Williams failed to qualify in the top three for Great Britain in the fourth heat of the 400m hurdles.
The Welshman could still make the semi-finals as one of the fastest losers once all the heats are completed. However, he struggled to get off to a bright start and started slipping behind midway around the track.
Puerto Rico's Javier Culson, who is arguably Dai Greene's biggest threat for gold, crossed the finish line first with a time of 48.33s.
Meanwhile, Greene got off to a positive start in his Olympic campaign, winning his heat with a time of 48.49s.
