Inge de Bruijn hit the pool for the first time at the World Championships, and eased into the 100 metres freestyle semi-finals by recording the sixth fastest time.
De Bruijn, who set world records in the 50 and 100 freestyle and the 100 butterfly as she won three gold medals at the Sydney Olympics, clocked a heat time of 55.65 seconds.
Johanna Sjoberg of Sweden and China's Xu Yanwei both recorded 55.44 to lead qualifiers in the 100 freestyle, followed by Katrin Meissmer of Germany in 55.51 and Australia's Sarah Ryan in 55.55.
Despite her Olympic success and her domination of the records, de Bruijn has never won an individual medal at the worlds.
"It's OK for now," the Dutch sprinter said of her opener. "I wasn't expecting a world record or anything, so I didn't go all out. It was only a morning race and it was my first race of the meet, so I feel pretty good."
Her compatriot Pieter van den Hoogenband upstaged Ian Thorpe in the men's 200 freestyle qualifying.
Van den Hoogenband, who won the 100-200 Olympic double in Sydney, advanced to the semis with a meet record of 1 minute, 47.18 seconds.
The Dutchman, who placed behind American Anthony Ervin in the 50 final on Monday, was just outside world record pace at the 100 turn but slowed over the last two laps.