Brendan Hansen held off the challenge of rival Kosuke Kitajima to defend his 100m breaststroke title at the World Championships in Melbourne.
Hansen said he tailored his training to focus on the Melbourne event
The American clocked a time of 59.80 seconds with Kitajima a fraction behind in second and Australia's Brenton Rickard back in third place.
Australia's Libby Lenton won the 100m butterfly ahead of Jess Schipper.
And Britain's Liam Tancock set a new Commonwealth record of 53.71 in the 100m backstroke heats.
"To set another British record is a real boost. Everything felt so good," said Tancock, who later qualified for Tuesday's final.
Fellow Brit Kirsty Balfour also reached the 100m breaststroke final after finishing second in a new British record of 1min 07.67secs behind Australia's Leisel Jones in her semi-final. Kate Heywood is also through to the final.
"Tonight was about just staying strong and going as fast as I could," said Balfour, who qualified fourth overall. "I'm really happy because my main event is the 200m, for me the 100m is just a bonus."
Anything is possible, but Thorpe's 200m freestyle world record is nearly perfect
Hansen, who became the most successful breaststroke swimmer in World Championship history, said he had been spurred on by the performance of Kitajima.
"We have a pretty healthy rivalry," said Hansen. "We make each other swim fast but we always shake hands at the end of the race. He brings out the best in me and I appreciate that.
"I'm not very pleased with the time but you don't break world records when you're in a battle like that."
American Michael Phelps believes Ian Thorpe's 200m freestyle world record - set in Fukuoka nearly six years ago - could be under threat in Tuesday's final.
Phelps, three-times Olympic champion Pieter van den Hoogenband of the Netherlands and South Korea's new 400m freestyle world champion Park Tae-hwan will be vying for gold after coming through Monday's semi-finals.
"There are a couple of guys who want to beat it tomorrow," said Phelps, who is chasing an unprecedented eight gold medals in Melbourne.
"Anything is possible, but Thorpe's world record is nearly perfect. There was a second between his first and second hundred. That's pretty much a dream-come-true 200 I would say."
Meanwhile, South Africa's Roland Schoeman took gold in Monday's 50m butterfly final, winning in a time of 23.28.
Lenton (left) held off Schipper to win the 100m butterfly
Schoeman knocked American Ian Crocker, the world champion and world record holder for 100 butterfly, into second place with Denmark's Jakob Schiott Andkjaer in third in from the outside lane.
And Lenton collected her second gold of the championships, following on from her success as part of Australia's 4x100m freestyle relay team on Sunday, winning in 57.15.
"I just wanted to go out and do my own race and back my own abilities," she said.
"They're all very good competitors but I'm also a pretty good competitor."
Elsewhere, Briton Dave Carry narrowly missed out on a place in the final of the men's 200m freestyle, while Simon Burnett failed to make the semi-finals.
In the women's 1500m freestyle Rebecca Cooke finished the heats in 10th and youngster Lizzie Simmonds came in 24th in the heats of the women's 100m backstroke.
British divers Tandi Gerrard (City of Leeds) and Hayley Sage (City of Sheffield) finished eighth in the women's 3m synchro pairing final on the final day of competition.
Gerrard and Sage finished with a total of 294.36 points. Gold went to Chinese divers Minxia Wu and Jingjing Guo with 355.80 points.
Russia's Anastasia Ivanenko has been banned for two years following a positive drugs test.
The 18-year-old Ivanenko was punished for using furosemide, a banned diuretic which can be used to mask steroids.
Ivanenko, who won gold in the 800m freestyle at last year's world short-course championships in Shanghai, tested positive in January and had been under a provisional suspension since 15 February.