Aaron Peirsol broke the 200m backstroke world record on Saturday, clocking 1 min 53.08sec to win the title at the US Swimming Championships.
Peirsol broke the previous world record of 1:53.94 set by Ryan Lochte in winning gold at the Beijing Olympics last August.
Lochte finished second in 1:54.21 and like Peirsol booked a berth in the event at the World Championships in Rome later this month. Tyler Clary was third in 1:54.53.
Peirsol was delighted with the swim and with regaining the world record he held from March 2002 until Lochte seized it at the 2007 World Championships.
Peirsol had equalled Lochte's mark at the US Olympic trials last July, before Lochte lowered the record again with hs victory in Beijing.
"We've been going back and forth with this for a few years now," Peirsol said of their rivalry. "It certainly helped me."
"This is a stepping stone for what's going to happen in a couple of weeks," Peirsol added of the upcoming World Championships. "He'll get a shot again."
But Peirsol was well aware that his swim was not the fastest ever, and that he'd have other swimmers to look out for in Rome, where swimming competition starts on July 26.
Japan's Ryosuke Irie, a 19-year-old student, clocked an astonishing 1:52.86 in a meeting in Canberra, Australia, on May 10.
FINA, world swimming's governing body, annulled the mark, which Irie achieved in a swimsuit that FINA determined had not been approved at the time.
"I'm elated I went that fast," Peirsol said. "But the real world record will come in the next two weeks."