Caeleb Dressel can win two golds on one morning in the Tokyo pool on Saturday while U.S. team mate Katie Ledecky hopes to complete an 800m freestyle hat-trick for her seventh career Olympic title.
World record holder Dressel is the clear favourite in the men's 100m butterfly, the only swimmer to qualify for the final in under 50 seconds -- his time of 49.71 the third fastest in history.
The sprint specialist, who has already won two golds at the Games to go with the two from Rio, then has a 50m freestyle semi-final before the mixed 4x100 relay closes out a busy morning on the penultimate day of pool action.
Ledecky has dominated the 800m freestyle since she won it as a 15-year-old at London 2012 and a third successive gold would make the world record holder only the fourth swimmer to have pulled off such a feat in any event.
One of the other three was U.S. medal machine Michael Phelps.
If she does win, it will be her sixth individual gold -- a record for a female Olympic swimmer -- and seventh in total. She is currently tied with Hungarian Krisztina Egerszegi on a record five individual titles.
Seven career golds would also leave her one behind compatriot Jenny Thompson's record eight for a female swimmer. All of those were from relays.
And she would be the first winner of the two longest events, having won 1,500m gold on its Games debut this week.
The 800m will be a final showdown with Australia's double gold medallist Ariarne Titmus, who beat Ledecky in the 200 and 400 freestyle. But Titmus has been well below Ledecky's pace in the 800.
Katie Grimes, Ledecky's 15-year-old team mate, starts in lane five.
The women's 200 backstroke is another Australia vs Team USA battle, with Emily Seebohm in lane four and fellow-Australian Kaylee McKeown in lane two with Americans Rhyan White and Phoebe Bacon in lanes three and five.
The mixed medley, featuring two male and two female swimmers, is making its debut as an Olympic medal event with British swimmers, set for their best Games in 113 years, fancying their chances after qualifying comfortably fastest.
"It's good for the Olympics. It shows what swimming can be," said 100m breaststroke champion Adam Peaty. "You need new races to keep it entertaining.
"We are feeling very strong. I have been racing with this team for a long time. We are looking forward to it and there's no pressure on us."