Budapest - The United States broke the mixed 4x100m medley world-record at the world aquatic championships on Wednesday while Cameron McEvoy was the fastest from the men's 100m freestyle heats.
The US quartet of Ryan Murphy, Kevin Cordes, Kelsei Worrell and Mallory Comerford clocked 3mins 40.28secs in the heats to break the previous mixed medley relay record of 3:41.71 set by Great Britain two years ago in Kazan.
The relay record is in danger of falling again in Wednesday
night's final with Australia, Great Britain and Canada all qualifying behind
Britain's Adam Peaty could also lower his own 50m
breaststroke record in Wednesday's final haven broken it twice on Tuesday in
the heats and again in the semi-finals.
Australia's McEvoy was the fastest from the men's 100m freestyle heats, clocking 47.97 secs, but all of the top eight were within a whisker of his time.
"That's been my quickest morning swim ever," said McEvoy.
"With the improvement in depth of the 100 free, you can't do a heat saying you're going to back off or not try here.
"You just have to nail it the way you want it and hope for the best."
Four swimmers, including Nathan Adrian of the United States, the 2012 Olympic champion, Britain's Duncan Scott and Japan's Shinri Shioura all clocked identical times of 48.46secs.
"It's really close, always has been, always will be," said Scott, who finished fourth in Tuesday's 200m freestyle final.
"The semis will be really close again, there won't be any backing off there, it'll be close tonight."
Japan's Kosuke Hagino, the Olympic 400m individual medley champ, was the quickest in the men's 200m IM heats, clocking 1:56.46.
His compatriot Daiya Seto, the 400m IM world champion was fourth fastest at 0.67 secs back.
Chinese teenager Qin Haiyang swam a new junior world record of 1:59.01 to qualify as eighth fastest for the evening semi-finals.
Following her 200m individual medley gold medal on Monday, home-crowd favourite Katinka Hosszu blasted her way into the women's 200m butterfly semi-finals as the fastest qualifier in 2:07.25.
China's Zhang Yufei was just behind at 0.25 secs back with compatriot Zhou Yilin at 0.47.
As they always do when a Hungarian swimmer enters the water, the crowd roared Hosszu home.
Hungarian backstroker Sara Joo said the home support is nothing like she has experienced before in Budpest.
"I have already swum in front of a home crowd at the European Championships two times, but this one has a completely different atmosphere. It was fantastic," she said after her heat.
China's Fu Yuanhui was fastest through the women's 50m backstroke heats into the semis.
The Olympic bronze medallist over 100m clocked 27.21 secs, just 0.15 off the world record.
Emily Seebohm, who won bronze on Tuesday when Canada's Kylie Masse broke the women's world record over 100m, was eighth fastest through at 0.70 behind Fu's time.
"I'm looking forward to the 50 semi tonight and hopefully making it through to the final, I'm not a specialist in this event and I'm having fun with that," said Seebohm.
Australia's Holly Barratt, a rookie at the world championships at the veteran age of 29, was the fifth fastest.