Katie Ledecky easily won the 400-meter freestyle at the United States swimming championships but she knows that she will need to do more to challenge for the title at the World Championships in Fukuoka, Japan next month.

"It wasn't great," Ledecky sighed after clocking four minutes and 0.45 seconds to lift the title in Indianapolis, Indiana.

She set a world record of 3:56.46 in winning gold at the Rio Olympics in 2016.

Since then, Australia's Ariarne Titmus has upset Ledecky for the 2019 world title and out-dueled her in the Tokyo Olympic final in 2021.

Titmus lowered the world record and Canadian teen Summer McIntosh lowered it again in March with a time of 3:56.08.

With Titmus having clocked 3:58.47 at the Australian trials this month, Ledecky was clearly dissatisfied with her time but not discouraged.

"So many things to learn from," the seven-time Olympic gold medallist and owner of 19 world titles said.

"So I'll try to learn as much as I can and be better in a couple of weeks."

Ledecky, who had opened the week with her fastest 800m free time since 2016, said she felt her rhythm was off from the start. With Fukuoka looming, she said, she'll focus on "making sure I'm ready to go when I dive in and build throughout the race."

Ledecky was still nearly three seconds ahead of runner-up Bella Sims, whose time of 4:03.25 was her best by more than three seconds and made the 18-year-old the seventh-fastest performer in the world this year.

Regan Smith grabbed her third title of the week with a victory in the 100m backstroke, clocking the second-fastest time of the year of 57.71 to hold off Katharine Berkoff, who punched her ticket to Fukuoka in 58.02.

"Super-happy with that time," said Smith, who also won the 200m backstroke and 200m butterfly this week as her training with coach Bob Bowman — former mentor to Michael Phelps — pays dividends.

In the men's 100m back, Hunter Armstrong pulled away on the final lap to beat four-time Olympic gold medallist Ryan Murphy with an impressive time of 52.33 seconds.

Murphy, who won double backstroke gold at the Rio Games, was second in 52.39 seconds.

World record-holder Lilly King rallied from third at the turn to win the 100m breaststroke in 1:04.75 with Lydia Jacoby, gold medallist at the Tokyo Olympics, second in 1:05.16.

Both King and Jacoby had to get past halfway leader Kaitlyn Dobler, who settled for third.