squeezed into the men's 400m hurdles final after finishing fourth in his semi-final.
With only the first two certain to progress from each of the three semi-finals, medal hopeful Greene struggled in a disappointing time of 48.19 seconds, and had an anxious wait to discover his fate.
But his time proved good enough to make the final as one of the two fastest outside the top two across all three races, although he will have an unfavourable lane in Monday's final.
Immediately after the race, Greene feared the worst, saying: "I can't believe it. I was going down the home straight and couldn't catch them.
"I don't know why I wasn't there. I'm devastated. I'm shocked and feel like I've let everyone down."
He will get an unexpected second chance to challenge for a medal, but
failed to make the final as he finished fourth in his race in 49.63secs.
European champion Williams looked in contention until stumbling off the last hurdle and said: "I gave it everything and I am gutted I am not going to get another chance to go out there because the support has been great.
"I had a dreadful last hurdle and lost all my momentum."
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