American teenager Katie Ledecky set a world record in the 400-meter freestyle at the U.S. National Championships on Saturday, while Michael Phelps struggled home in sixth in the 100 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 1,500m freestyle world records at the same time since Janet Evans, took her latest record-breaking performance in stride.