Katie Ledecky of the United States admitted she was shocked after breaking her own 1,500m freestyle record in the heats of the world swimming championships in Kazan on Monday.

The 18-year-old, who also holds the 400m and 800m records, clocked 15min 27.71sec to better the 15:28.36 secs she swam at the Pan-Pacific Games in August last year.

"My coach told me to swim the first 900m easy, build over the next 300, then the final 300 was my choice and to be honest, it felt pretty easy," said Ledecky, who came within a second of her 400m record on Sunday.

"I wasn’t kicking much and I think breaking the world record is testament to the hard work I have put in and the shape I am in right now.

"I am pretty shocked that I was able to do that."

Ledecky only recently finished high school in Maryland, but this is the eighth time the teenager has set a world record in her fledgling career.

It is the fourth time the defending champion has broken the 1,500m record alone having first achieved the feat in Barcelona two years ago on her way to winning the world title.

- Sun eyes second gold -

This is the second world record to fall in Kazan after Sweden's Sarah Sjostrom feat in the women's 100m butterfly heats on Sunday.

Also in Monday's heats, Chinese medal-machine Sun Yang stayed on course to claim the second of the four golds he wants in Kazan after cruising into Monday night's 200m freestyle semi-finals as the fastest qualifier.

Sun defended his 400m freestyle title on Sunday, but faces some stiff competition over the shorter distance.

Britain's James Guy, who took silver behind Sun in the 400m final, was just 0.10sec behind Sun's time of 1min 46sec dead.

Germany’s world record-holder Paul Biedermann and Australia's Cameron McEvoy are also medal contenders for Tuesday's final at 0.39sec behind Sun's time.

Missy Franklin of the United States opened her individual campaign as the world and Olympic 100m backstroke champion was fifth fastest into Monday's semi-finals.

Hungary's 'Iron Lady', Katinka Hosszu, was the fastest through in a national record of 58.78sec but will sit out the evening's backstroke semis to defend her 200m individual medley world title.

She finished second to Australia's Emily Seebohm in her heat having won gold to Seebohm's silver at the 2012 Olympics and Barcelona's worlds in 2013.

- breaststroke battles -

After Australian medal hopes Mack Horton and Christian Sprenger bombed in their respective heats and the men's team failed to qualify for Sunday's 4x100m freestyle final, Mitchell Larkin was fastest through to the evening's 100m backstroke semi-finals.

He clocked 52.50sec, an Oceania record, just ahead of world and Olympic champion Matt Grevers.

"Yesterday was a bit of a wake-up call for us," admitted Larkin.

"It's all about keeping up with our females now," he said pointing to the Australian women's relay gold success.

There's a battle brewing in the women's 100m breaststroke.

Russia's Yuliya Efimova was the fastest through to Monday's semi-finals after her 16-month ban for using steroids expired in February.

Lithuania's defending Olympic and world champion Ruta Meilutyte, China's Shi Jinglin, Jessica Hardy of the USA and Japan's Kanako Watanabe all posted fast times.

Monday night's 100m breaststroke final pits Britain's world-record holder Adam Peaty against South Africa's Olympic champion Cameron van der Burgh and the pair have broken the championship record three times in Kazan.
Source: AFP