Irvine - World record-holder Katie Ledecky cruised to victory in the 800m freestyle at the US swimming championships on Wednesday to lock down a berth for the Pan Pacific Championships next month.

Ledecky, owner of the top 10 times ever posted in the event, was under world record pace through 350m, but settled for a comfortable victory in 8:11.98 - well outside the world record of 8:08.79 she set at the Rio Olympics in 2016.

"Usually it takes me a few races to get into these meets, so (I) knew that was going to be the case probably," said Ledecky, whose four victories in Rio took her tally of Olympic gold medals to five.

The 21-year-old American was confident she would find more speed before the Pan Pacs in Tokyo from August 9-14.

"I felt pretty smooth throughout," she said.

"I never really was able to change gears. I'll get back to work and find ways to be better in a couple of weeks."  

While Ledecky was easing into the meet - a qualifier not only for Pan Pacs but also for next year's world championships in South Korea - Hali Flickinger capped a career day with a victory in the 200m butterfly.

Flickinger, 24, sliced almost a full second of her lifetime best in the heats with a time of 2:05.87.

She broke the US Open record of 2:05.96 set by Mary T. Meagher in 1981 in becoming the third-fastest US woman ever.

She couldn't quite reproduce that in the final, but she held on to win in 2:06.14, with Katie Drabot second in 2:07.18 and Regan Smith third in 2:07.42.

"I definitely think it is a step in the right direction," Flickinger said.

I definitely know there's more in the tank. That just showed me that it's going to come."

Blake Pieroni, 22, stormed home to beat 2012 Olympic champion Nathan Adrian in the 100m freestyle in 48.08.

Adrian, who also owns four Olympic relay golds, was philosophical.

"In a race like that you're just racing for places," said Adrian, who was second in 48.25 and like Ledecky thought he'd find more speed heading into Tokyo.

"Usually I do get a little faster with that extra two weeks of rest," he said.

Townley Haas was third in 48.30 and Zachary Apple fourth in 48.34. Caeleb Dressel, who won the 100m freestyle as part of his seven-gold haul at the world championships in Budapest last year, finished sixth in 48.50.

He was entered in seven more events this week, so still had plenty of chances to make the Pan Pacs, where he could yet secure a 100m freestyle berth at the 2019 worlds.

Adrian said Dressel, who inked his first sponsorship deal with Speedo last week, was adjusting to his departure from the US collegiate amateur ranks at the right time, with the 2020 Tokyo Olympics still two years away.

"It's certainly not the best that we've seen from Caeleb (but) he's going to be down to 47-low as early as next year," Adrian said.

World and Olympic champion Simone Manuel notched a dominant victory in the women's 100m freestyle in 52.54, with Mallory Comerford’s second in 53.09 and Margo Geer third in 53.44.

Justin Wright surged from seventh place at the halfway stage - fifth at the final turn - to win the men's 200m butterfly in 1:54.63.

Zach Harting was second in 1:55.11 and 16-year-old Gianluca Urlando and Jack Conger tied for third in 1:55.21.

Jordan Wilimovsky, already headed to Pan Pacs in open water, won the men's 1 500m freestyle in 14:48.89 - fifth fastest in the world this year.