| Kosuke Kitajima en route to the 100m breaststroke gold in Barcelona on Monday. (Reuters)
Barcelona: Michael Phelps claimed the fourth world record of the world swimming championships with a magnificent swim in the semi-finals of the menís 200 metres butterfly on Tuesday.
The 18-year-old American came home in one minute 53.93 seconds to lop more than half a second off the 1:54.58 mark he set at the last world championships in Fukuoka on July 24, 2001.
Three world records fell on Monday.
Matt Welsh set the first in achieving a surprise victory in the menís 50 butterfly final, fellow Australian Leisel Jones posted the second in the womenís 100 breaststroke semi-finals and Japanís Kosuke Kitajima made it three in the menís 100 breaststroke final.
Phelps, who will swim the 200 butterfly final on Wednesday, will also contest the 100 butterfly and 200 and 400 individual medley as well as two relays in Barcelona.
Earlier, Ian Thorpe saw off the challenge of his old rival Pieter van den Hoogenband to retain his 200 metres freestyle title in convincing style at the world swimming championships on Tuesday.
The victory over his Olympic conqueror took the 20-year-old Australianís record aggregate of world gold medals to 10 and brought him his second Barcelona title after he won an unprecedented third 400 freestyle gold on Sunday.
Sloveniaís Peter Mankoc led down the first length, with the Dutchman equal second and inside the world record split, with Thorpe fifth.
But Thorpe had overtaken his rival by the 150-metre mark and surged clear win by more than a second in 1:45.14.
Van den Hoogenband took the silver in 1:46.43 and Australiaís Grant Hackett the bronze in 1:46.85.
Meanwhile, Germanyís Hannah Stockbauer scooped her second gold medal of the championships with a runaway victory in the womenís 1500 metres freestyle.
The defending 1500 champion, who won the 400 freestyle on Sunday, retained her title handsomely, winning by more than nine seconds from 17-year-old American Hayley Peirsol.
Peirsol took over second place from fellow American Diana Munz just after the halfway mark and held off a late challenge from 29-year-old former East German Jana Henke.
Stockbauer won in 16 minutes 00.18 seconds, a European record and tantalisingly close to making her the second woman after American Janet Evans to break 16 minutes.
Peirsol took the silver in 16:09.64 and Henke, 1991 world bronze medallist and 1992 Olympic bronze medallist in 800 freestyle, the bronze in 16:10.13.
The Chinese sweep continued as Luo Xuejuan scored a comprehensive victory over new world record holder Leisel Jones to become the first woman to retain the 100 metres breaststroke title at the championships.
Luo, double breaststroke world champion in 2001, charged down the first length inside world record schedule, turning in 30.87, and none of her rivals could get close as she took the gold in one minute 6.80 seconds.
American Amanda Beard, double Olympic silver medallist in 1996, took the silver in 1:07.42 and 17-year-old Australian Jones, the 2000 Olympic and 2001 world silver medallist, had to settle for bronze in 1:07.47 - more than a second slower than the world record 1:06.37 she set in Mondayís semi-finals.