Adam Peaty set an early benchmark but home hope Daiya Seto suffered a shock on a busy first day of swimming at Tokyo 2020, one that saw Australian duo Brendon Smith and Emma McKeon shine.
Peaty is aiming to become the first British swimmer to defend an Olympic title having taken gold at Rio 2016, while Seto made an inauspicious start to the Games.
Here is a round-up from Saturday's opening heats at the Tokyo Aquatics Centre.
EASY PEATY! RECORD HOLDER SAFELY THROUGH
Peaty has not been beaten in a 100m breaststroke race since 2014 and is aiming to lower his own record time of 56.88 seconds, which was set at the World Championships two years ago.
The dominant 26-year-old posted a 57.56 in a solid start to his Games, qualifying fastest ahead of Dutch rival Arno Kamminga (57.80).
Peaty produced the eighth fastest time in history, while Kamminga set a Dutch record and is the only other swimmer who has broken 58 seconds in the event.
WOE FOR SETO AS SMITH STARS
These Games were meant to be about redemption for home medal hope Seto, the bronze medallist in a men's 400m individual medley race won by compatriot Kosuke Hagino four years ago.
Seto was the favourite in the race and seeking to atone after being barred last year after his involvement in an extramarital affair. He won world gold in the 400IM two years ago, but he sensationally failed to make it out of the heats at these delayed Olympics.
Seto, who returns to the pool next in the 200m butterfly on Monday, said: "It hurts and I'm frustrated at myself. It's my mistake and I have to owe up to it. What's done is done and not a whole lot I can do about it."
By contrast, it was Smith who qualified fastest, clocking a 4:09.27 to set a new Australian and Oceanic record. New Zealander Lewis Clareburt was second quickest, winning a tight third heat over 2016 silver medallist Chase Kalisz.
Smith's compatriots Elijah Winnington and Jack McLoughlin both made it through the men's 400m freestyle, with Germany's Henning Muhlleitner (3:43.67) fastest, just ahead of Austrian Felix Auboeck (3:43.91).
Brendon Smith is the first Australian to go under the 4:10 mark in the 400m IM— Australian Swim Team (@DolphinsAUS) July 24, 2021
With the time of 4:09.27, he shaved off over half a second of the previous record set at the Aus Trials #Tokyo2020 | #TokyoTogether | #ItsTokyoTime | @AUSOlympicTeam pic.twitter.com/bmOKb9tviO
MCKEON, ZHANG SET THE MARK
There was more Aussie promise in the women's 100m butterfly – a seemingly wide-open event with three competitors having gone under 56 seconds this year.
McKeon – a winner of four medals in Rio, including relay gold in the 4x100m – and China's Zhang Yufei were two of those to achieve the feat and both recorded a 55.82 to top the timesheets in the heats, the former setting an Australian record for good measure.
Sarah Sjostrom, who holds the world record, was not even certain to race in this event after fracturing her elbow earlier this year, but posted an impressive 56.18 to go third quickest. The Swede was ahead of American Torri Huske (56.29), the third woman who has gone under 56 seconds this year.
Australia were also dominant in the women's 4x100m freestyle, their time of 3:31.73 almost two seconds better than the Netherlands.
Teenager Emma Weyant bettered her personal best with a 4:33.55 to comfortably post the best time in the women's 400 individual medley prelims. The 19-year-old - a winner at the U.S. Olympic trials - was almost two seconds quicker than Great Britain's Aimee Willmott (4:35.28).
Back-to-back AUSSIE RECORDS! This time to @emma_mckeon in the Women's 100m butterfly.— AUS Olympic Team (@AUSOlympicTeam) July 24, 2021
Her time of 55.82 is a new Oceania Record, and was a dead heat with China's Yufei Zhang. #TokyoTogether | #Swimming pic.twitter.com/weqOsPZ3IF