Former world 800 metres champion Caster Semenya failed to reach the Olympic qualifying time when she clocked two minutes 3.60 seconds in her return to action after injury on Saturday.
Semenya won her event but the time was well outside the London Games qualifying standard of 1:59.90.
"I ran a little stupidly because I didn't take the lead from the beginning. I thought they would push the pace but we all learn from our mistakes," she told reporters.
"Everyone knows the Olympics is the most important thing this year so I must set the qualifying time, and we'll take it from there."
Semenya, 21, was embroiled in a controversy over her gender after her 2009 triumph in Berlin and the International Association of Athletics Federations ordered her to undergo gender testing.
In July 2010 she was cleared to run in women's events and is currently returning to full fitness after a back problem disrupted her season last year.
