Russia''s Yulia Efimova booked her place on Wednesday in the final of the women's 200 metres breaststroke but her nemesis Lilly King of the United States failed to make it past the semi-finals.
Australia's Taylor McKeown was the fastest qualifier for Thursday's final, followed by Japan's Rie Kaneto and Britain's Molly Renshaw, with Efimova sixth and King 12th. Only the top eight make the final.
With two past doping suspensions, Efimova, bronze medallist in the event in London 2012, was initially excluded from the Rio Games but succeeded in a last-minute appeal last week to be allowed to compete.
Comments by King describing her rival as a drug cheat set up a tense clash in Monday's 100 breaststroke final, in which the American defeated the Russian.
The two swam in separate semi-finals on Wednesday.
King's forthright comments earlier in the week gave an even sharper edge to traditional US-Russian rivalry.
Russian swimming chief Vladimir Salnikov told Reuters on Tuesday the atmosphere surrounding his team reminded him of the Cold War.
King, 19, told reporters earlier on Wednesday that she had no regrets for what she said.
"My parents raised me to say what I wanted to say even if it wasn't what people wanted to hear necessarily, so it's always been like how I am," she said.
"And I'm going to stick with my guns."