Tuesday's women's 100m backstroke final will be a battle of the record breakers with Australia's Kaylee McKeown going for gold against American Regan Smith and Canada's Kylie Masse.
The trio are the three fastest swimmers in the history of the event, all having held the world record, with McKeown the current owner.
They showed their form in Sunday's heats with Masse setting an Olympic record before Smith took it from her and then McKeown went even faster.
In the women's 100m breaststroke, American Lilly King faces a tough challenge to the defence of her title with South African Tatjana Schoenmaker in the prime lane four after qualifying fastest.
World record holder King's old Russian foe Yulia Efimova is also in the final.
The pair made headlines at Rio 2016, when King wagged her finger at the previously suspended Russian and branded her a drug cheat. That clearly still rankles Efimova.
"I'm rooting for Tatjana Schoenmaker," Efimova told Russian media. "If I don't win myself, then let Tatjana win."
The men's 100m backstroke features defending champion and world record holder Ryan Murphy of the United States going for a fourth career gold with Russian Kliment Kolesnikov looking dangerous and Australian Mitch Larkin also a threat.
The first final of the morning is the men's 200m freestyle with Britain's Duncan Scott and Tom Dean hoping to medal and American Kieran Smith in the lane between them.