Katie Ledecky may finally have a true challenger.

The U.S. swimmer Katie Ledecky’s domination is so routine that her surname has become a verb, synonymous with crushing the competition. For nearly a decade, she has ledeckied away in her specialties — distance swims longer than 400 meters — rarely facing a true challenger and certainly nothing resembling a rival.

Now she has one.

Ariarne Titmus of Australia, a fearless Tasmanian who talks big and has the speed in the pool to back it up, is about to ask Ledecky the one question she has never had to answer in her two previous Olympic appearances: How will she respond to a swimmer who has placed a target on her back and taken dead aim at it?

“I’m sure she is going to be fast, and I’m sure she thinks the same of me,” Ledecky, 24, said of Titmus in a pre-Olympic news conference this month.

How fast is Titmus? Lately, when it has counted most, she has been a good bit faster than Ledecky at both 200 and 400 meters, races that Ledecky swept four years ago.

At Australia’s Olympic trials last month, Titmus, 20, missed breaking Ledecky’s world record of 3:56.46 in the 400 by just half a second. At the U.S. trials, also in June, Ledecky swam the distance in 4:01.27.

In the 200, Titmus came within 0.11 of a second of the record, which was set in 2009, when swimmers wore sleek suits that reduced drag, which are now banned. Ledecky swam the 200 freestyle at the U.S. trials in 1:55.11, more than two seconds behind the world record.

Aside from her times, Titmus’s comments after the trials rocketed across the swimming world.

“She’s not going to have it all her own way,” Titmus said of Ledecky after her 400 race.