Australia's Ariarne Titmus defeated five-time Olympic gold medalist Katie Ledecky of the United States to grab gold in a thrilling women's 400-meter freestyle final on Monday.
Britain's Adam Peaty repeated his gold medal success from Rio in the 100-meter breaststroke while Canadian Maggie MacNeil stormed to victory in a close women's 100-meter butterfly.
Ledecky has been the dominant force in women's swimming but in the first of three head-to-head showdowns with Titmus at these Games it was the Australian who triumphed with a superbly timed swim.