Sprint machine Caeleb Dressel blasted a 21.32 seconds 50m freestyle heat in his bid to earn the world's fastest swimmer title Friday, while 100m champion Emma McKeon set a new Olympic record to top the women's timesheets.
The explosive Dressel has been untouchable over the splash and dash for the past two seasons and is overwhelming favourite in Tokyo, with Brazilian Cesar Cielo's 2009 world mark of 20.91 within reach.
The 6ft 3ins American has won two gold in Japan and will go for a third in the 100m butterfly on Saturday, with the 50 final on the last day of competition Sunday.
"You certainly can't let off the gas too much," he said. "I knew where I was in that race, I didn't want to expend more energy than I needed to. It's about managing your energy mentally and physically."
Dressel didn't suit up for the 4x100m medley relay heats that followed, but could still swim the final on Sunday.
He is also in with a chance to compete in the 4x100 mixed medley relay decider on Saturday, leaving a possible six gold medals on the table if he chooses to do so.
Dressel admitted it had been hard to put the emotion of winning 100m freestyle gold on Thursday behind.
"I had to recoup pretty fast, there's a lot of emotional energy that goes into these meets physically, but emotionally it'll drain you quicker than the physical," he said.
"I tried to have a nap (today) but couldn't so emotions are still high."
Just three men have won the Olympic 50m title twice -- Alexander Popov, Anthony Ervin and Gary Hall Jr -- and France's Florent Manaudou is the only one in Tokyo with a chance to join them.
But he hasn't been under 21.50 since the 2016 Olympics, where he came second to Ervin and then retired and switched to a handball career. He came back to the sport in 2019 and clocked 21.65 to be second behind Dressel, with Greek Kristian Gkolomeev third.
McKeon returned to the pool after storming to the 100m crown in the morning session, carrying her electric form into the evening.
She backed up with 24.02 in her heat, a new Olympic record.
McKeon, team-mate Cate Campbell and Dutch 2012 Olympic champion Ranomi Kromowidjojo are the only women to crack 24 seconds this year, making them top contenders.
Campbell, who won the 100m bronze, was third fastest into the semi-finals in 24.15, with defending champion Pernille Blume of Denmark second best in 24.12.
World record holder Sarah Sjostrom came fourth with Kromowidjojo eighth.
Blume denied she was feeling any pressure as the title holder.
"No, that's five years ago. It doesn't matter what you have done or haven't done. We are all here on an equal basis," she said.
Distance star Gregorio Paltrinieri would ordinarily be a favourite in the men's 1500m as defending champion and second fastest ever behind China's Sun Yang.
But the Italian was struck down with glandular fever in June, hampering his built-up.
He qualified only third in his heat and fourth overall, behind Ukrainian Mykhailo Romanchuk (14:45.99), American Robert Finke, the 800m champion, and Germany's Florian Wellbrock.
After China upset them in world record time to win the 4x200m freestyle relay, Australia and the United States were keen to make amends in the women's 4x100m medley heats.
