The US Olympic swimming trials produced a brace of backstroke world records - for Aaron Peirsol and Natalie Coughlin - and two more Beijing Games berths for Michael Phelps on Tuesday.

World and Olympic champion Peirsol lowered his own 100m backstroke world record with a time of 52.89sec to set up a defense of the title he won in Athens.

Aaron Peirsol swims to a win and setting a new world record time of 52.89 seconds in the men's 100m backstroke at the US Olympic swimming trials in Omaha, Nebraska on Tuesday. Reuters

Coughlin broke the women's 100m back world record for the second time in two days, punching her ticket to Beijing with a victory in 58.97sec - the first woman to swim the event in less than 59 seconds.

They were the fifth and sixth world records to be set in three days in the temporary pool at the Qwest Center as American swimmers put on a formidable display of strength.

Peirsol said his performance was just a measure of the standard required to make the US Olympic team.

"It's a huge confidence booster," he said. "Any time I can come out on top in a heat like that - that's the fastest heat I've ever been in," said Peirsol, who was followed home by Matt Grevers (53.19).

"Sixth place would have gotten a gold medal last time," noted Peirsol, who won gold in Athens in 54.06.

Ryan Lochte was third, followed by Randall Bal, whose semifinal leading time of 53.09 was the third-fastest ever.

"Aaron, he's one of the best competitors out there," Phelps said. "His last 15m, that's where he wins races, he can finish a race better than anyone out there."

Coughlin, too, had to battle as the women's 100m back world mark fell for the third time in two days.

Coughlin had lowered the mark to 59.03sec in the final heat on Monday, after Hayley McGregory had set a world record in the previous heat of 59.15.

One day removed from her world record, McGregory couldn't make the Olympic team. She finished third behind 200m backstroke world champion Margaret Hoelzer, whose time of 59.21 matched the world record Coughlin held in the event heading into the meet.

"It just feels so good to be on the team," said Coughlin, who went on to book her spot in the 200m medley final later. "You've got to take a breath and enjoy the rest of the meet."

Phelps continued his progress toward an assault on Mark Spitz's record of seven gold medals at one Olympics with a victory in the 200m freestyle, securing his berth in the individual event and the 4x200m free relay.

Phelps clocked 1:44.10, with Peter Vanderkaay finishing second in 1:45.85.

While Phelps didn't match his world record of 1:43.86, the times of the top four swimmers signaled the fierce challenge the US will offer in the relay in Beijing.

"I'd probably go out on a limb and say we could put together the fastest four guys ever," Phelps said.

Ricky Berens was third in 1:46.14 and Klete Keller fourth in 1:46.20, putting all four within the top nine performers all time.

With Lochte, who skipped the 200m free final to focus on the 100m back, the US will have at its disposal five of the top nine 200m free performers ever in the relay.

"That's going to be an exciting event," Phelps said.

Phelps, who set a world record in the 400m medley on Sunday, also moved a step closer to qualifying for a fourth Beijing event with the fastest time in the 200m butterfly semifinals.

"Honestly, I felt really good in the 200 fly," he said of the semi-final swim that came about 45 minutes after the 200m free. "I wanted to have a swim that set me up for something good tomorrow night. I'm not exhausted by any means."

Jessica Hardy recovered from a disappointing semifinal swim to win the 100m breaststroke in 1:06.87.

Hardy said nerves were to blame for a sub-par performance on Monday, but she was able to steady herself and secure her Olympic place.

Megan Jendrick, the 2000 Olympic gold medalist at Sydney who said she only started serious training for the 100m two months ago, gave herself a chance at a second bite at Olympic glory with the second-fastest time of 1:07.50.

Katie Hoff, who like Phelps is two-for-two in finals so far this week, kept her multi-medal challenge on track with the fastest semifinal times in both the 200m freestyle (1:57.10) and the 200m medley (2:09.94).

(AFP via China Daily July 3, 2008)