GWANGJU, South Korea (REUTERS) - Caeleb Dressel showed no signs of slowing down at the world swimming championships on Wednesday (July 24) after the American delivered the year's fastest time in the 100-metre freestyle heats.

Dressel, who two years ago matched Michael Phelps' record of seven world titles at a single championships, has already pocketed gold in the 4x100m relay and the 50m butterfly in Gwangju and there will be plenty more opportunities to add to his haul.

"It felt good. That was kind of how we wanted to swim it - just out controlled and then come home well," said the 22-year-old, who finished in a time of 47.32.

"You've got to get better each race so same thing with the fly, just got to improve on different parts of the race going through the meet. It's a long meet and I've got to pace it."

Russian Vladislav Grinev was second fastest, 0.60sec behind Dressel, while Olympic champion Kyle Chalmers qualified in 10th place for the semi-finals in 48.66.

South African Chad le Clos and Britain's Duncan Scott, who caused uproar on Tuesday by refusing to shake China's Sun Yang on the podium, were missing from their 100m freestyle heat. Sun is swimming under a cloud in Gwangju with the Court of Arbitration for Sport (CAS) set to hear a doping case against him in September.

China's Fu Yuanhui tabled the quickest time in the women's 50m backstroke heats, the 2015 champion touching in a time of 27.70, 0.15 second ahead of Brazil's Etiene Medeiros, who beat Fu to the title in Budapest two years ago.

In the women's 200m butterfly, American Hali Flickinger went through as the top seed in a time of 2:05.96, ahead of Hungarian duo Boglarka Kapas and Liliana Szilagyi.

In the last event of the early session, the United States topped the mixed 4x100m medley relay (3:41.23) ahead of Australia and Russia.