Katie Ledecky celebrates her win and world record time in the women's 400 meters freestyle final during the US National Championships on Saturday (US time) in Irvine, California. Photo: AFP

American teenager Katie Ledecky set a world record in the 400 meters freestyle at the US National Championships in Irvine, California on Saturday (US time), while Michael Phelps struggled home in sixth in the 100 meters backstroke.

The 17-year-old Ledecky clocked 3 minutes 58.86 seconds to better the previous record of 3:59.15 held by ­Italy's Federica Pellegrini. The Italian's mark was set in 2009 before high-tech polyurethane swimsuits were banned.

Ledecky, who became the first American to hold the 400, 800 and 1,500 meters freestyle world records at the same time since Janet Evans, took her latest record-breaking performance in stride.

"Honestly, I didn't think about it too much," the 2012 Olympic 800 meters freestyle champion told NBC sports. "I just wanted to put together a good swim and go a best time. I did, so I'm happy."

Ledecky, the reigning 400 meters world champion, came close to the world record with a time of 3:59.89 in the morning heats.

She went out hard in the final and was under record pace at halfway by more than a second when she turned at 1:57.72.

When Ledecky touched at 300 meters more than 1.5 seconds ahead of Pellegrini's mark, a world record seemed almost a formality.

She slowed over the next lap to hit 350 meters in 3:29.41, just 0.38 seconds ahead of world record pace, but it was enough of a ­cushion and she held on to touch the wall 0.29 faster than Pellegrini's mark.

Phelps, meanwhile, made a slow start and was never in contention in the 100 meters backstroke as Olympic and world champion Matt Grevers won in 52.75 seconds, the third fastest time in the world this year.

On the comeback trail after coming out of retirement, Phelps was more than a second behind in 53.95 though given the event has never been one of his strongest, and he has never contested it at the Olympic Games, his performance was not a huge surprise.

"Whatever I really put my mind to, I know I can accomplish," said Phelps. "I'm not going to use the excuse of taking off [nearly two years]."

In the most significant meet since Phelps launched his comeback in May, the 18-time Olympic champion ­finished seventh in the 100 meters freestyle on Wednesday and second in the 100 ­meters butterfly on Friday.

In the women's 100 meters backstroke, ­Missy Franklin clinched her fifth straight national title in 59.38, which ranks her third in the world this year.