Natalie Coughlin and Aaron Peirsol blitzed to world records in the 100 meters backstroke to highlight another breathless day of action at the US Olympic swimming trials on Tuesday.

Coughlin sparked the record demolition when she stormed to the wall in a time of 58.97 seconds, trimming 0.06 off her own mark to become the first woman to dip under 59 seconds.

It was the third time in two days the record had been lowered, with Hayley McGregory and Coughlin swapping the mark in successive preliminary heats on Monday.

Even though McGregory briefly held the record, her third- place finish behind Margaret Hoelzer in the final was not good enough to earn a trip to Beijing [Images] with only the top two securing Olympic berths.

"It just feels so good to be on the team," Coughlin told reporters. "It is such a cruel meet.

"I have so many friends I'm cheering for but there are only two spots.

"People assume if you're the world record holder that you're going to go to Beijing no problem.

"But anything can happen at this meet, it's the nature of the beast.

"It's unfair but I've seen both sides of it and having so many of those experiences helped me through tonight."

With the crowd still buzzing over Coughlin's swim, Peirsol brought them to their feet again by powering home in 52.89 seconds to improve his record of 52.98 with the help of one of the strongest fields ever assembled in the event.

"It's a huge confidence boost anytime I can come out on top in a race like that," said Peirsol, who like Coughlin swam in the controversial hi-tech Speedo LZR racer bodysuit. "It's the fastest heat I've ever been in...by far.

"Sixth place would have gotten a gold medal last time.

"I knew what I had to do today, I knew 54.0 wasn't going to make the team this time."

Six world records have now tumbled in the first three days of the trials.

NARROWLY MISSED

Michael Phelps narrowly missed rejoining the world record blitz but clinched his second Olympic berth by cruising to victory in the 200m freestyle in 1:44.10, the third fastest time ever.

Less than 40 minutes later, the six-times Olympic gold medallist was back in the spotlight, clocking the top qualifying time of 1:54.02 in the 200m butterfly.

"That was by far the fastest 200 metre free field I've ever been in, including the world and Olympics [Images]..everything," said Phelps, who is targeting Mark Spitz's record of seven gold medals at a single Summer Olympics in Beijing.

In the night's other final, Jessica Hardy booked a ticket to her first Olympics by winning the 100m breaststroke in 1:06.87 ahead of 2000 Sydney gold medallist Megan Jendrick.

Katie Hoff, who has claimed a world record in the 400 medley and two Olympic spots, looks poised to add to more events to her Beijing program after posting the top times in the 200m freestyle and 200m individual medley.