Aussie teen sensation shocks the world

Australia's Ariarne Titmus has shaken up the swimming world order, upsetting American great Katie Ledecky to claim the 400m freestyle gold at the World Swimming Titles in South Korea.

The 18-year-old becomes Australia's newest world champion with a startling race.

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

Her time of 3:58.76 was also a new Australian record as she beat Ledecky by 1.21 seconds and denied the American a fourth straight 400m world title.

"I knew that I properly had that in me," Titmus said.

"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."

Ledecky is so dominant in women's long distance races of 400m, 800m and 1500m, she hadn't lost a race since 2012 over the distance.

A 14-time World Champion, Ledecky was left stunned with NBC Sports reporting she was still floored during the medal ceremony.

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 after burners 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.

The world was floored by it's new world champion.

Ledecky said the Aussie had a "heck of a swim" and said she wasn't used to racing people who can swim a time under four minutes.

"I just got to the last turn and felt like I just tightened up, my legs were just dead and obviously Ariarne took advantage of that and had a heck of a swim," she said.

"Obviously this stings a little, it's unfamiliar and different, and I need to rebound from this and get my fight back.

"I felt like I stayed pretty calm throughout the race, felt like I made a move at the right time, but my legs and arms didn't have it at the end. Just have to move on and focus on the next races."

- with AAP