Katie Ledecky’s Unlikely New Role: Underdog?

Katie Ledecky at the U.S. Olympic swimming trials in Omaha last month.Credit...Hiroko Masuike/The New York TimesTOKYO — For nearly a decade, Katie Ledecky has lived a mostly blessed athletic existence.

As a painfully shy 15-year-old, she upset a defending gold medalist and world-record holder to win her first Olympic title at the 2012 London Games. She has had barely a slip since.

Her domination is so routine that her surname has become a verb, synonymous with crushing the competition. And 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…