Field athletes Sunette Viljoen and Willem Coertzen also acquitted themselves admirably in the javelin and decathlon events respectively on Thursday, though neither made it onto the podium.
The 21-year-old Semenya had to sit out track and field for almost a year after her stunning world victory in Berlin in 2009 after undergoing gender tests, but came back to win silver in the Daegu worlds.
In her semi on Thursday, Semenya sat in fifth at the bell but kicked with 250m to go to finish ahead of Russian Elena Arzhakova.
"I think the time I ran makes me very confident. You have to think about your own race, you have to think of yourself," said Semenya, who clocked 1:55.45 to win in Berlin three years ago.
"I was a bit nervous but this crowd makes me feel at home and reminds me of good memories. It's about putting on the spikes and then just running."
Janeth Jepkosgei, the silver medallist in the Beijing Games who won bronze in Daegu, was third but went through as one of the two faster finishers outside the two automatic spots from each of the three semis.
Jepkosgei will be joined in Saturday's final by teammate Jelimo, who won her heat in 1:59.42 ahead of another Russian, Ekaterina Poistogova.
A third, and arguably the strongest, Russian made the final in the shape of world champion Mariya Savinova, who won her semi in 1:57.42 ahead of Burundi's ever-improving Francine Niyonsaba in a national record of 1:58.67.
The last finalist will be Alysia Montano of the United States, who finished fourth in Semenya's fast second heat.
Javelin: Sunette Viljoen
Javelin thrower Sunette Viljoen finished fourth in the woman's javelin final at the London Olympic Games on Thursday night.
Viljoen was in second position after her first attempt of 64.53 meters, but failed to better the throw and slipped down the list as Germany's Linda Stahl took the bronze medal (64.91 meters).
Stahl finished behind fellow German, Christina Obergfoll (65.16 meters) and the Czech Republic's winner, Barbora Spotakova (69.55 meters).
Decathlon: Willem Coertzen
Willem Coertzen finished nineth overall in the men's decathlon at the London Olympic Games on Thursday night.
Coertzen, who holds the SA record in the event, finished on 8173 points, 696 behind American winner Ashton Eaton.
Eaton finished ahead of fellow countryman Trey Hardee (8671 points), while Cuba's Leonel Suarez (8523 points) came third.
The South African ended day one of the event in 11th place and made a superb start on Thursday, winning his 110m hurdles heat in a new personal best (PB) time of 14.15 seconds. – Sapa-AFP