TOKYO—Australia’s Ariarne Titmus defeated five-time Olympic gold medallist Katie Ledecky of the United States to grab gold in a thrilling women’s 400m freestyle final on Monday. Britain’s Adam Peaty repeated his gold medal success from Rio in the 100m breaststroke while Canadian Maggie MacNeil stormed to victory in a close women’s 100m 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. The American was 0.66 seconds ahead at the halfway point but Titmus had closed that gap to 0.16 seconds by the 300m turn and grabbed the lead at the 350m mark and never looked back. “I can’t believe it, I’m trying to contain my emotions,” said Titmus, who praised her rival for raising the bar in the sport. “I thanked her, I wouldn’t be …