Semenya, a bronze medallist in the 1500m in London as she attempted an audacious double, clocked 2min 01.33sec.
“I just want to keep winning,” said the South African, dogged by gender accusations since shooting to fame when she won world gold in Berlin in 2009.
“That’s all I’m concentrating on. I’m feeling okay after the 1500m, so I’m ready to go again.”
Defending world 800m champion Maryna Arzamasova of Belarus could only finish fourth in her heat, out of the automatic three qualifiers from each of the six heats.
Kenya’s Margaret Wambui, bronze medallist at both the world indoors and Olympics last year, won that heat, but Arzamasova’s season’s best of 2:01.92 was not fast enough to see her bag one of the faster loser places.
Wambui’s teammate Eunice Sum, world champion in Moscow in 2013 and bronze medallist in Beijing two years later, was a late withdrawal.
Francine Niyonsaba of Burundi, world indoor champion and Olympic silver medallist, also qualified with heat victory in 1:59.86.
Friday’s semi-finals are scheduled for 1835 GMT, with the final set for 1910 GMT on Sunday.