BARCELONA, Spain (AP) – Two Australians and one Japanese swimmer set world records Monday at the world championships.

The three world marks fell in about 50 minutes at Palau Sant Jordi on a day when Jenny Thompson won the gold medal for the United States in the 100 butterfly.

Matthew Welsh established a world record in winning the 50-meter butterfly gold. Leisel Jones set the mark in the women’s 100 breaststroke semifinals. Kosuke Kitajima broke the record in capturing the gold medal in the men’s 100 breaststroke.

Welsh covered his single swim down the pool in 23.43 seconds, beating the record of 23.44 set by fellow Australian Geoff Huegill at the last worlds in Fukuoka, Japan. Afterward, Welsh looked over at Huegill, who finished fourth a few lanes down, with his arms wide apart in apparent disbelief.

“I had no pressure in that race being a backstroker,” Welsh said.

Ian Crocker of Portland, Maine won the silver in 23.62, with the bronze going to Evgeni Korotychkine of Russia in 23.73.

Barely 10 minutes after Welsh set his mark, Jones was timed in the second semifinal heat of the 100 breaststroke, beating the record of 1:06.52 set in 1999 by Penelope Heyns of South Africa.

“I think it was seeing Matt set a world record that inspired me,” Jones said.

Kitajima was timed in 59.78 in the 100 breaststroke. The Japanese eclipsed the record of 59.94 set by Roman Sloudnov of Russia in Fukuoka in 2001. Brendan Hansen of the United States took the silver in 1:00.21, with the bronze going to James Gibson of Britain in 1:00.37.

Thompson, an eight-time Olympic champion, captured her first major individual gold since 1998. All eight of her Olympic titles have come in relays.

Thompson’s time of 57.96 seconds beat silver medalist Otylia Jedrzejczak of Poland (58.22) and bronze medalist Martina Moravcova of Slovakia (58.24).

“It’s great to come back after a year and a half off and have this kind of success, so I’m really happy,” said Thompson, who recently completed her second year of medical school and is planning to break from her studies to compete in next year’s Athens Olympics.

“It was very close and I put my head down for the last four strokes, which may not have been smart,” she added.

Yana Klochkova of Ukraine won the women’s 200 individual medley in 2:10.75, the third fastest time ever. Alice Mills of Australia claimed silver in 2:12.75 and Zhou Yafei of China took the bronze in 2:12.92.

Thompson’s teammate Natalie Coughlin finished eighth out of eight swimmers in the 100 butterfly final.

Earlier, Coughlin failed to qualify for the 100 backstroke semifinals. The American swam 1:03.18 in the backstroke, more than three seconds off her world record-time of 59.58 seconds set last year.

Coughlin, who said she has been ill since Saturday, apparently was weakened by her strong performance in the opening leg of the Americans’ gold-medal effort in the 400 freestyle relay Sunday night. The Californian has entered seven events.

“Thankfully, I have a couple of days off,” Coughlin said.

Australian star Ian Thorpe (1:47.20) finished second to Dutchman Pieter van den Hoogenband (1:46.32) in the semifinals of the 200 freestyle.

Antje Buschulte of Germany led the women’s 100 backstroke semifinals in 1:00.61, and Aaron Peirsol of the United States was fastest in the men’s semifinals in 54.28.

AP-ES-07-21-03 1733EDT