American superstar Caeleb Dressel has delivered the fastest time 50m freestyle time in the world this year with a blistering 21.29 seconds win at the US swim trials in Greensboro, North Carolina.

Olympic veteran Dressel, who added six golds to his coffers in Tokyo, was in top form again after besting the field in the 50 and 100 butterfly and 100 freestyle earlier in the competition.

"I thought I could be 21.40 tonight so to see 21.29, I'm pretty fired up," he said.

The 25-year-old will lead a relatively inexperienced US men's team at next month's world championships in Budapest and said he was still adjusting to his role as one of the squad's elder statesman.

"I'm one of the old guys on the team now, which is really weird," he said.

"I still feel young, I still feel good."

Katie Ledecky added the women's 400 free to her program for Budapest after the seven-time Olympic gold medallist cruised to the win in 3:59.52.

Ledecky bested her gold medal effort in Tokyo by more than three seconds, having already won the 800 on the opening night of the competition followed by the 400 and 200.

Regan Smith punched her ticket to Budapest by breaking her own record with a 57.76 first place finish in the 100m backstroke.

Hunter Armstrong added the 100m back to his schedule for worlds with a 52.20 finish.

The night before, the 21-year-old claimed a world record in the men's 50m back with a time of 23.71, trimming 0.09 off Russian Kliment Kolesnikov's previous mark.

The world championships will run from June 18 to July 3 in Budapest.