The South African, looking as if she was out for a Sunday morning jog, won the third of three semis in the fastest time of the night - one minute, 58.15 seconds - with Britain's Lynsey Sharp chasing her over the line to also advance.
Semenya has dominated the 800m this season and there has even been speculation that she could take down Czech Jarmila Kratochvilova's 1983 mark of 1:53.28 seconds - the longest standing athletics world record set in an era when eastern European doping was rife.
"The pace was a bit quicker than I thought but I just went with the flow," Semenya said before playing down the world record prospects.
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"Winning the race is what I'm focused on, my pb is still quite new, so I just need to have a look at the video, see how I can improve and see how it goes in the final."
Tall, loping 20-year-old Margaret Wambui, surprise winner of the Kenyan trials this year, looks as if she could be a medal challenger, as could Burundi's Francine Niyonsaba, the world indoor champion, after the two went through together from the first semi.
If the final comes down to a sprint finish Pole Joanna Jozwik will also be in the mix after she came from last to first in the final 60 metres to win her heat ahead of impressive Canadian Melissa Bishop.
Belarusian world champion Marina Arzamasova went through as a fast loser.
Russian duo Mariya Savinova and Ekaterina Poistogova, first and third in the London Olympics four years ago, were unable to take part in Rio due to the doping suspension on the country's athletes and have both been recommended for life bans by the World Anti-Doping Agency.
(Editing by Ed Osmond)
