TOKYO, July 25 (Reuters) - Katie Ledecky can become a six-times Olympic gold medallist on Monday if the American wins her long-awaited 400m freestyle showdown with Australia"s world champion Ariarne Titmus. The big duel of the Tokyo pool will be one highlight of a morning that is also likely to see Adam Peaty take the 100m breaststroke gold and make history as the first British swimmer to defend an Olympic title. "It’s going to be a great race with Ariarne. I’m really excited to be in that first final tomorrow," said Ledecky, who won four of her golds in Rio in 2016 and the other in London in 2012. The American is the defending 400m champion and world record holder and goes into the final with the fastest time from the heats. Titmus, 20, also won her heat comfortably enough and said: "I"d like to think I"ve got a bit more in the tank for the final." Ledecky would once have been the day"s nailed-on certainty for gold but that role has been taken by world record holder Peaty, whose times so far have been faster than any other swimmer has ever managed in the event. The United States will again be in the medal hunt later in the morning after qualifying second fastest behind Italy for the 4x100 freestyle relay, with Australia third. The women"s 100m butterfly could be a big moment for China"s top medal hope Zhang Yufei, fastest in the semi-finals, but France"s Marie Wattel and Australian Emma McKeon will be pushing hard. Sweden"s world record holder and reigning champion Sarah Sjostrom, American Torri Huske and Canadian Maggie MacNeil are also very much in contention. Sjostrom had surgery for a broken elbow after slipping on ice in February and a medal would be a remarkable turnaround in her fourth Games. "When I started swimming again after my surgery, I was like, "I’m never, ever going to be able to swim again." I couldn’t physically take a stroke in the water," she said after Saturday"s heats. "I couldn’t do anything. So, I am super proud to do this comeback."