Elizabeth Beisel sailed into the final of the women's 400 metres individual medley on Saturday when she set the fastest time in the heats.
The American, who is looking to add the Olympic gold to the world title she won in Shanghai last year, easily won her heat in a time of four minutes 31.68 seconds.
China's Ye Shiwen was second quickest, just 0.05 behind Beisel, setting the stage for an epic final that could rival the highly-touted showdown between Michael Phelps and Ryan Lochte.
Australia's Stephanie Rice, the defending Olympic champion and world record holder, snuck into the final in seventh place after saving her energy for the final.
"There are some really hot times," said Rice.
"These are races where you have to be quick to make the final because everyone wants to be in there, but at the same time you have to try and conserve something because it is such a taxing race."