Rio de Janeiro: Racing nothing but the clock, Katie Ledecky gave the U.S. its first victory by crushing her own world record in the 400 freestyle. The result was totally expected. The unassuming teenager from suburban Washington has dominated the longer freestyle events since winning gold in the 800 free at the London Olympics as a 15-year-old. The only drama was whether she’d take the world record even lower. Her powerful stroke quickly made that a moot point, too.