Great Britain's Lynsey Sharp failed to qualify for the women's 800m Olympic final after a seventh-place finish in her semi-final.
European silver medallist Sharp
ran 2:01.78 as defending champion Pamela Jelimo of Kenya won Thursday's first heat in 1:59.42.
South Africa's 2009 world champion Caster Semenya ran a season's best of 1:57.67 to win the second semi-final.
"That first round was something Lynsey was able to enjoy, but what I think she might be disappointed with is not being able to run her personal best."
Current world champion Mariya Savinova claimed the final heat in 1:58.57.
Semenya booked her place in Saturday's final as
the fastest qualifier,
just ahead of Russia's Elena Arzhakova [1:58.13] who finished second in the same heat.
But it was a disappointing end for Sharp, 22, who was
chosen to represent Great Britain ahead of four other athletes who had run faster qualifying times.
"I got in a really bad position on the back straight and on the second lap it was just a wall in front of me," an emotional Sharp told BBC Sport. "It just wasn't a very clever race."
Meanwhile, USA recorded the fourth fastest time in history to qualify for Friday's
women's 4x100m relay
The quartet of Tianna Madison, Jeneba Tarmoh, Bianca Knight and Lauryn Williams won their qualifying heat in 41.65 seconds, just 0.05 outside of the Olympic record set by East Germany in Moscow 1980.
Ukraine held off Jamaica, who were hampered by a messy final-leg handover, on the line to take the second heat in 42.36 seconds.