INDIANAPOLIS: Aaron Peirsol overshadowed Michael Phelps' first appearance at the US National Swimming Championships on Wednesday, reclaiming his 100m backstroke world record with a stunning swim.

Peirsol clocked 51.94 seconds, slicing .44 seconds off the mark set by Spain's Aschwin Wildeboer Faber in the lead off leg in a 4x100m medley relay at the Mediterranean Games on July 1.

"When my world record was broken it was a total surprise," Peirsol, who has now broken the record six times, told reporters. "My goal coming in was to break the world record and that was added motivation, most definitely.