The FINA World Championships in Gwangju, Korea have their first big upset on the first day of swimming competition.
Five-time Olympic champion Katie Ledecky (3:59.97) suffered a shock defeat in the 400m freestyle and was beaten by Ariarne Titmus on Sunday. The 18-year-old Australian caught the American favourite on the final lap on Sunday to win in Oceania record time of 3:58.76 and deny the American a fourth straight 400m world title.
USA's Leah Smith took bronze in 4:01.29.
It was only Ledecky's second defeat in a world championship final.
Ledecky was leading going into the final lap but she ran out of gas and had the slowest last 50m in the eight-woman field.
Titmus showed why she is nicknamed "Terminator" going a staggering 1.83 seconds faster than the 22-year-old American.
"I knew that I properly had that in me," Titmus said after her stunning victory. "I feel pretty normal, it was just a swimming race."
"There was no pressure really coming into this race. I just wanted to fight as hard as I could. In the last 50m I gave it everything."
Ledecky had clinched every major 400m freestyle title since 2013 including the Olympics and world championships.
Speaking after the loss, she said: "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.
"Obviously this stings a little, it's unfamiliar and different, and I need to rebound from this and get my fight back."
This is just the beginning for Ledecky in Gwangju where she is scheduled to compete in five events.
Next up for her is the 200m freestyle on Wednesday in which Titmus is one of her her main rivals again.