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American swimmer Katie Ledecky’s first test at the Tokyo Olympic Games is upon us.

And, if you haven’t heard yet, there seems to be something — or someone — in the water.

Enter Ariarne Titmus of Australia. The 20-year-old tops this season’s world rankings in the 400m freestyle, clocking in at 3:56.90, which she swam in June at the Australian Swimming Trials. As it stands, her time currently marks the second-fastest swim in history. The fastest? Ledecky.

At the U.S. Olympic trials, also in June, Ledecky swam the distance in 4:01.27. The fastest time she’s posted in this event in 2021 was 3:59.25 in April.

After Titmus stunned Ledecky to take gold in the 400 meter final at the FINA World Championships in 2019, headlines flashed everywhere with one word: underdog?

Rewind back to the 2016 Rio Olympics. Ledecky was unstoppable in the 200m freestyle, 400m freestyle and 800m freestyle distances. The then-20-year-old Ledecky swam to a world record 3:56.46 in the 400m, winning the gold medal by a margin of 4.7 seconds over Great Britain’s Jazz Carlin.

But this year’s Olympics will not be so easy for the American.

For Ledecky, now 24, anything other than a gold medal sweep in the distance events will feel like she’s come up short. Ledecky’s rival, however, has much less to lose — there’s no shame in being beat by a five-time Olympic champion. And in the 200m and 400m, this could give Titmus the edge.

If one thing is clear, it’s that Tokyo could be a historically epic showdown between Ledecky and Titmus.

Ledecky will swim in heat three of the 400 meter freestyle on Sunday morning (EST), with Titmus in heat four. The final will be Sunday night (EST).