Swimming-China's Sun, American Ledecky light up world championships

* Ledecky also wins third world 400m freestyle title

* Swede Sjostrom sets 100 freestyle world record in relay

By Rod Gilmour

BUDAPEST, July 23 (Reuters) - China's Sun Yang destroyed the field to win a hat-trick of men's 400 metres freestyle titles while Katie Ledecky led the way again for the United States as the swimming got underway at the world aquatics championships on Sunday.

Sun posted a world-leading time of three minutes 41.38 seconds as he eased to victory by more than a body length over bitter Australian rival Mack Horton.

Horton beat Sun to the Olympic title last year in Rio de Janeiro, with the Australian calling the Chinese a "drug cheat" before the Games in reference to Sun having served a three-month ban in 2014 after testing positive for a banned stimulant.

Sun said at the time that the stimulant was in medication to treat a heart issue and did not enhance his performance.

In a race billed as 'War on the Water, Part 11', the rivalry in the pool failed to materialise on Sunday as Sun become only the second swimmer to win a third 400 freestyle world title after Ian Thorpe of Australia 14 years ago.

Sun, wearing a distinctive pink swimsuit, looked in ominous form after taking the lead at the 150m mark and having claimed the gold medal he glared over at Horton, who was racing in lane two after a sluggish morning heat.

The Australian had to settle for silver, with Gabriele Detti of Italy taking bronze.

LEDECKY TREBLE

Thirty minutes later, Ledecky won the 400m freestyle in another one-woman show of speed and agility.

The 20-year-old college swimmer's championship record time of 3:58.34 secured a trio of world titles as she became the first woman to do so in any discipline.

Leah Smith ensured a United States one-two, while Li Bingjie of China touched for bronze.

Ledecky also helped the U.S to a brilliant gold in the women’s 4x100m relay -- which included a world record for Swede Sarah Sjostrom for the 100m freetyle -- as the American quartet ruined Olympic champions Australia’s title defence.

Leading off the Swedes, who finished fifth, Sjostrom posted 51.71 seconds, smashing the time set by Australian Cate Campbell last year by 0.35 of a second.

A frenetic end to the evening’s action saw the U.S. regain top honours in the men’s 4x100m freestyle relay.

The quartet of Calaeb Remel Dressel, Townley Haas, Blake Pieroni and Nathan Adrian edged Brazil into second place.

Katinka Hosszu is favourite to follow Ledecky into the record books, judging by her form in the 200m individual medley semi-finals.

The Hungarian, roared on by 12,000 fans inside the Duna Arena, qualified fastest for Monday's final in a time of 2:07.14.

Adam Peaty of Britain showed that he has the potential to lower his world record mark of 57.13 seconds after easing into the men's 100m breaststroke final. (Reporting by Rod Gilmour; Editing by Christian Radnedge and Ken Ferris)