Four world records were set on Tuesday and fast racing was plainly evident that morning
World records continued to fall at the FINA World Championships as the United States quartet in the mixed 4x100 metres medley relay lowered the mark in blistering fashion on Wednesday.
Four world records were set on Tuesday and fast racing was plainly evident the morning as the US team of Ryan Murphy, Kevin Cordes, Kelsi Worrell and Mallory Comerford posted three minutes 40.28 seconds ahead of the final later on Wednesday.
The US time smashed the mark set by Britain at the 2015 championships in Kazan, Russia by 1.43 seconds as they qualified nearly four seconds ahead of the field.
The mixed medley relay was added to the world championships two years ago and in June the International Olympic Committee put the event into the Tokyo 2020 Games programme.
"It's one of the most exciting races to watch," said Murphy, who led off in the backstroke.
Comerford led the quartet home in the mixed gender event and Murphy said the Olympic addition was "exciting for the sport."
He added: "A lot of people like watching individual medley because there are a lot of lead changes, but when you add in guys and girls it makes it even bigger."
Australia's Oceania record time saw them qualify in second, with world champions Britain posting the fourth-best time.
Australia's otherwise quiet world championships were given a lift when Cameron McEvoy's bid for a first global title in the men's 100-metre freestyle began in energetic style.
McEvoy, the world silver medallist, topped qualifying and was the only man under 48 seconds in the preliminary heats, posting a punchy 47.97 seconds.
"With the improvement and the depth of the 100 freestyle, you have to nail it the way you want it and hope for the best," said McEvoy, who finished seventh in Rio.
Rising American star Caeleb Dressel qualified in third with 48.26 seconds.
Elsewhere, Japan's Kosuke Hagino began his bid to become the first non-American winner of the men's 200-metre individual medley since 2001 with an authoritative display in the heats.
Michael Phelps and Ryan Lochte dominated the event from 2003 but Olympic silver medallist Hagino is clearly intent on upgrading his 2013 world silver after opening with a time of one minute 56.46 seconds.
China's domination in the women's 50-metre backstroke continued when current champion Fu Yuanhui clocked a swift 27.21 seconds to lead qualifying.
Local favourite Katinka Hosszu of Hungary also produced an efficient swim in the women's 200-metre butterfly to head into the semi-finals as leading qualifier.