KAZAN, Russia (Reuters) - American teenager Katie Ledecky started her bid to defend her three world titles by qualifying fastest for the women's 400 meters freestyle final on the opening morning at the world championships in Russia.
The 18-year-old recorded four minutes, 01.73 seconds to qualify fastest for the final later on Sunday, 2.74 seconds ahead of Australia's Jessica Ashwood.
Ledecky is competing in four freestyle events in Kazan, adding the women's 200m freestyle to her program over a grueling eight-day schedule.
"It was good to finally race and get a first swim under the belt," said Ledecky. "I think I was pretty much all on adrenaline so hopefully I can focus a little more tonight.
"I probably went out too fast which I have been told a 1,000 times not to do on the first 65 meters."
In the final individual heat of the morning session, Adam Peaty of Great Britain set the meeting's first championship record as the 20-year-old recorded the fastest time in the 100m men's breaststroke heats.
After 50m, Peaty was 0.01 seconds off his own world record pace, before easing off the gas in the closing stages ahead of the semi-finals.
"That felt relaxed," he told reporters. "I've done a lot of work on that front end, keeping it as relaxed as possible."
Earlier, Sun Yang, attempting to win four individual titles in Kazan, began his quest in style, winning his 400m men's freestyle heat to qualify fastest for the final later on Sunday.
Sun is defending titles over 400m, 800m and 1500m, with the Chinese giant also competing over 200m, as well as the relays.
Racing in a world-class heat, Sun posted 3:44.99 as the Chinese overtook Britain's James Guy in the closing stages.
Guy, a threat to Sun's title aspirations, recorded 3:45.37 while Ryan Cochrane of Canada qualified third fastest for the final in 3:45.86.
Australia's Mack Horton, however, failed to make the final.
World champion Katinka Hosszu cemented her status as favorite for the women's 200m individual medley title, easing through to the semi-finals as fastest qualifier.
Hosszu ducked well beneath her time when she won the title two years ago, winning her heat in 2:07:30. Britain's Siobhan O'Connor recorded 2:08.82 to qualify second quickest.
In the women's 100m butterfly, Sweden's Sarah Sjostrum, the defending champion, qualified fasted for the semi-finals.
The United States failed to reach the final of the men's 4x100m freestyle relay for the first time at a world championships after they finished 11th in qualifying.
The quartet had previously reached the world podium in every championships, except at Fukuoka in 2001.
Editing by Sudipto Ganguly