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Katie Ledecky edged Australian arch-rival Ariarne Titmus Sunday to be quickest in the 400m freestyle heats, setting the scene for an epic battle for gold, on a Tokyo night that saw two Olympic records fall.
The American great kickstarted a gruelling schedule, which will also see her tackle the 200m, 800m and 1500m freestyle, by touching in 4mins 00.45secs at the Tokyo Aquatic Center, where heats are in the evening and finals each morning.
Titmus, who stunned the Olympic champion at the 2019 world championships and has been posting red-hot times this year, cruised through the next heat in 4:01.66, with both keeping plenty in the tank for Monday's final.
"It's gonna be a great race with Ariarne and a lot of great swimmers in that field," said Ledecky, who won gold in the 200m, 400m, 800m and 4x200m free relay in Rio in 2016.
She added that she didn't pay attention to times, after being told how Titmus went.
"I mean we were (always) going to be there in the middle lanes in all likelihood, it'll be a great race tomorrow."
Titmus threw down the gauntlet at the Australian trials last month, with a sizzling 3:56.90, the second fastest of all-time and just outside Ledecky's world record 3:56.46.
"I felt really good, probably the most relaxed I've felt in a while," said the Australian, who will also challenge Ledecky in the 200 and 800.
"Tonight was really about trying to conserve your emotions for tomorrow morning."
- Cleared the nerves -
Fellow Australian sensation Kaylee McKeown got her maiden Olympic campaign off to a flying start with a storming 100m backstroke heat.
The 20-year-old smashed Regan Smith's world record last month, the only Olympic distance mark to fall this year, and, given her form, is considered the gold medal favourite.
She touched in a new Olympic record time of 57.88, just minutes after Smith had also lowered the mark to 57.96, with her swim following Canadian Kylie Masse doing the same with her 58.17.
The previous Olympic best of 58.23 had not been bettered since 2012 with the woman who held the time until Sunday, Australia's Emily Seebohm, still racing and in contention.
"To get out there and put up a really good swim cleared out the nerves so hopefully moving onwards and upwards," said McKeown ahead of the semi-finals on Monday.
"I put the pressure on myself to be honest. I like the nerves, it means you're going to do something special and you care about what you're doing."
Another young gun, Russian prodigy Kliment Kolesnikov, was under world record pace until easing up in the final 20m of the 100m backstroke heats to signal his intentions.
The 21-year-old, who holds world junior records in the 50, 100, and 200m backstroke, topped the timesheets in 52.15, outgunning American defending champion Ryan Murphy who cruised in 53.22.
Elsewhere, world record-holder and defending champion Lilly King won her 100m breaststroke heat in 1:05.55 and was never in trouble.
But she faces a battle to retain her title after South Africa's Tatjana Schoenmaker lowered King's Olympic record time by touching in 1:04.82.
King denied feeling any pressure from being favourite.
"I felt like that last time (in Rio) even though a lot of other people didn't so I'm kind of riding the wave I've been on for the last five years and hope it will turn out alright," she said.
South Korea's Hwang Sun-woo set a new national record 1:44.62 to lead the way into the men's 200m freestyle semi-finals, while Italy were fastest into the men's 4x100m freestyle final with a 3:10.29.
They were marginally ahead of the United States, without superstar Caeleb Dressel, and the Kyle Chalmers-led Australia.