Semenya carried South Africa's flag at the opening ceremony in London and is a leading medal contender. She ran her heat in 2:00.71, behind the 2:00.47 run by Alysia Johnson Montano of the United States.
The rest of the Olympic action Wednesday:
The U.S. men's volleyball team got off to a strong start at the London Olympics, but it's headed home without a medal.
The defending Olympic champions lost 28-26, 25-20, 25-20 to Italy in the quarterfinals.
U.S. setter Donald Suxho bent over with his hands on his knees and shook his head as Italy celebrated the win at Earls Court. And after shaking hands with the Italians, the Americans turned to applaud the many U.S. fans in the crowd.
Dragan Travica and captain Cristian Savani each had four aces for Italy, which will face Brazil in Friday's semifinals. Savani finished with 19 points.
Russia takes on Bulgaria in the other semi.
CYCLING - BMX
Two spectacular crashes marred the start of the BMX competition, and time trial world champion Caroline Buchanan of Australia and Raymon van der Biezen of the Netherlands posted the best times of the seeding runs.
On the challenging course sitting next to the London Velodrome, American rider Brooke Crain crashed in the home stretch in a section made of a succession of small jumps. She managed to get back on her bike to cross the finish line but officially was listed as not finishing. If she's able to continue, she would be given the 16th and final seed.
Crain was a late replacement on the U.S. team for Arielle Martin, who was hospitalized after a crash during a final training run on July 30 in California.
Latvian rider Edzus Treimanis also went down on the track at the same spot, falling face first.
The U.S. was shut out of Olympic sailing medals for the first time since 1936 when women's match racing skipper Anna Tunnicliffe was beaten in the quarterfinals by Finland.
Tunnicliffe, who was born in England and moved to the United States when she was 12, was the only remaining American with a chance for a medal.
While the Americans underperformed, the Australians continued to pile up victories.
The 49er crew of Nathan Outteridge and Iain Jensen sailed three laps of honor to collect the gold medal they had clinched two days earlier.
Middleweight Claressa Shields dominated Kazakhstan's Marina Volnova in the semifinals of the first Olympic women's tournament, earning a spot in the title bout against Russia's Nadezda Torlopova.