The 18-year-old, who won women's 400m freestyle gold on Sunday, recorded 15 minutes 27.71 seconds to lower her previous mark, set at the Pan Pacific Championships in August 2014, by 0.65 seconds.

Ledecky was inside world record pace throughout her preliminary heat as the American finished 28.81 seconds ahead of her nearest competitor, Jessica Ashwood, the Australian who qualified third fastest for Monday's final. It was the fourth time that Ledecky has broken her own 1500m freestyle record. She first set the mark at the 2013 world championships, smashing compatriot Kate Ziegler's previous time from 2007 - 15:42.54 - by over six seconds.

"To be honest, it felt pretty easy," said Ledecky. "I wasn't kicking that much and breaking that record is testament to the work I have put in and the shape I am in right now. "I was barely even focusing on this morning's swim as I was so relaxed. I realised just towards the end as I could see people waving and I knew where my parents, brother and uncle were sitting."

Earlier, Katinka Hosszu, the 2014 world swimmer of the year, continued to set personal records after two days of competition, the Hungarian posting the fastest qualifying time in the women's 100 metres backstroke heats. Hosszu, 26, recorded a Hungarian national record time of 58.78 seconds, though she has opted to focus on the women's 200m individual medley final later on Monday rather than contest the 100m backstroke semi-finals.

Hosszu's omission has opened the door for both Emily Seebohm, the Australian who posted the second fastest qualifying time, and America's Missy Franklin, who advanced through in fifth place, with 59.59 seconds, her fastest time of 2015.

"Emily and I were joking that we have never been next to each other in prelims, only in finals," said Franklin, winner of six gold medals at the 2013 Barcelona world championships. In the men's 200m freestyle, China's Sun Yang and Britain's James Guy qualified fastest, the morning after winning gold and silver respectively in the 400m freestyle.

Sun posted 1:46.00, while Ryan Lochte, the 30-year-old American, also secured a semi-final berth, qualifying 13th quickest. Yuliya Efimova, competing in her first major competition since her doping ban ended in February, topped preliminary qualification in the women's 100m breaststroke.

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