TOKYO: US star Katie Ledecky will seek revenge against Australian rival Ariarne Titmus in the Olympic pool on Wednesday as shockwaves from the dramatic withdrawal of gymnastics star Simone Biles from the team competition rippled across the sporting world.
Titmus, nicknamed "Terminator", toppled 2016 champion Ledecky in a thrilling 400m freestyle race and she leads the timings heading into the morning 200m final, with the American third fastest.
Hong Kong's Siobhan Haughey was second-quickest, and the eye-catching final also includes long-standing world record-holder Federica Pellegrini of Italy.
Ledecky, who won four gold medals at the 2016 Rio Games, is in the middle of an arduous programme that also includes the 800m as well as the 1500m, with the final of that event taking place later on Wednesday morning.
The American enjoyed a welcome evening off after the 200m heats and hopes it will give her some extra energy in Wednesday's final, held in the morning to suit US TV audiences.
"I think that's all I need to get myself a little reset going into that next morning," she said.
Hungary's defending champion Katinka Hosszu is seeking to make up for a miserable start to the Olympics in the women's 200m individual medley, though she qualified second-slowest.
Medals are also on offer in the men's 200m butterfly and the men's 4x200m freestyle relay.
Tuesday's action in Tokyo was overshadowed by the withdrawal of Biles from the women's team final, with the superstar gymnast's participation in the rest of the Games now uncertain.
The 24-year-old American arrived in Japan as one of the headline acts of the pandemic-postponed 2020 Games, shouldering an immense weight of expectation as she pursued a record-equalling nine Olympic titles.
But she came into the team finals after uncharacteristic stumbles in qualifying.
The US quartet were pipped to gold by the Russians who claimed the Olympic women's team title for the first time since the 1992 Barcelona Games.
After she collected her silver medal -- Biles was eligible having at least started the final -- the four-time Olympic champion confirmed her shock exit was due to her psychological state.
"I have to do what's right for me and focus on my mental health and not jeopardise my health and my wellbeing," Biles said, with her teammates gathered beside her.
"I just don't trust myself as much as I used to, and I don't know if it's age. I'm a little bit more nervous when I do gymnastics."
Support for Biles poured in from around the world.
"Gratitude and support are what @Simone_Biles deserves," tweeted White House spokeswoman Jen Psaki. "Still the GOAT (greatest of all time) and we are all just lucky to be able to see her in action."
Filipino boxing legend Manny Pacquiao tweeted: "Once a champion, always a champion. God Bless @Simone_Biles."
Home fans are also coming to terms with the defeat of Japanese tennis star Osaka, who was playing her first event since withdrawing from the French Open citing mental health issues.
Fresh from leading the Russian gymnasts to a narrow men's team win over Japan, world champion Nikita Nagornyy will again face home hope Daiki Hashimoto in the men's all-around competition.
In men's basketball, the USA will look to bounce back quickly against Iran after they fell to a strong French team for their first Olympic defeat in 17 years.
The collection of NBA players led by Kevin Durant looked far from a "Dream Team" and they will come up against a motivated Iran following decades of political ill-feeling between the two countries.
Elsewhere, three-on-three basketball -- a version of the sport designed to appeal to a younger audience -- will celebrate its first Olympic champions and baseball, a highly popular sport in Japan, will make its return to the Games schedule for the first time since 2008.
And the men's rugby sevens competition reaches its climax with defending champions Fiji playing Argentina and New Zealand tackling Britain in the semi-finals.
Tropical Storm Nepartak made landfall in northern Japan's Miyagi region early on Wednesday.
Local authorities have issued non-mandatory evacuation orders but Olympic organisers have said events due to be held with spectators in the region on Wednesday would go ahead, including men's football.