Titmus, 18, powered past five-time Olympic champion Ledecky on the final lap on the opening night of the eight-day titles at Gwangju.

She clocked three minutes, 58.76 seconds to beat Ledecky by 1.21 seconds, denying the American a fourth straight 400m world title.

Remarkably Ledecky - a 14-time world champion - had not been beaten over 400m, 800m or 1500m freestyle at a major international meet since 2012.

"I feel pretty normal, it was just a swimming race," Titmus said after her stunning win.

"There was no pressure really coming into this race. I just wanted to fight as hard as I could - in that last 50m I gave it everything."

Titmus led through the first 200m and at one stage was under world-record pace before Ledecky made her move.

The American hit the front with 250m left and looked set to power home before Titmus put on the afterburners in a stunning final lap finish.

"I knew I had to take it out hard to be with her. Her third 100m is still a lot better than mine but suddenly I felt something," Titmus said of her final surge.

Ledecky finished second in 3:59.97 - well off her world record of 3:56.46 set at the Rio Olympics.

American Leah Smith took bronze in 4:01.29.

Australian Associated Press