Canada's Summer McIntosh took gold in the women's 400m freestyle event at the World Aquatics Championships in Singapore as American Katie Ledecky finished with the bronze.
The event was seen as a battle between 18-year-old McIntosh - who has three Olympic gold medals - and 28-year-old nine-time Olympic champion Ledecky.
However, McIntosh led from the start to win in three minutes 56 seconds, with China's Li Bingjie second and Ledecky 2.23secs behind the leader.
McIntosh has a chance to join Michael Phelps as the only swimmer to win five individual titles at a single world championships as she is also set to race in the 400m and 200m individual medley, 200m butterfly and 800m freestyle.
Speaking to Canadian broadcaster CBC, the teenager said: "It's amazing, day one is probably my biggest day in the meet.