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Mark J. Terrill, Associated Press
Natalie Coughlin swims to a new world record time at 59.03 during the women's 100-meter backstroke preliminaries at the US Olympic swimming trials in Omaha, Neb., Monday.

OMAHA, Neb. — Natalie Coughlin stood on deck watching Hayley McGregory erase her world record in the 100-meter backstroke. Coughlin decided she wanted it back.

Two minutes later, it was hers again.

Coughlin reclaimed the mark by winning her preliminary heat in 59.03 seconds Monday at the U.S. Olympic swimming trials.

One heat earlier, McGregory finished first in 59.15, bettering Coughlin's mark of 59.21 set Feb. 17 in a prelim heat at the Missouri Grand Prix.

"I don't think you ever expect to go a world record, but it happened," McGregory said. "Really cool that it's possible to happen in the prelims."

She had barely climbed out of the pool before Coughlin got in and electrified the crowd again.

"The original goal is just to make top 16 and conserve, but I really didn't want her to have it for very long," Coughlin said.

McGregory didn't. Her glory lasted 2 minutes until Coughlin reasserted her dominance as the defending Olympic champion. Coughlin won five medals at the 2004 Athens Games.

Their efforts made Coughlin and McGregory the two leading qualifiers for the evening semifinals, which involve 16 swimmers. The back-to-back records were the 41st and 42nd out of 46 that have been set by swimmers wearing Speedo's LZR Racer suit since its debut in mid-February.

McGregory, a former Texas swimmer who transferred to Southern California, just missed making the 2004 Olympic team when she finished third in the 100 back at trials. Only the top two in the final earn berths.

Michael Phelps and Ryan Lochte dueled in the same prelim heat of the 200 freestyle, and the finish was the same as Sunday night's 400 individual medley final: Phelps first, Lochte second.

"He just told me not to do what I did last night and look at him underwater on the last wall," Phelps said. "It's always fun getting in the water and racing him."

Phelps took the lead on the final turn for home and touched in 1 minute, 46.63 seconds on his 23rd birthday. Lochte was next in 1:46.83.

Peter Vanderkaay and Erik Vendt, who both train with Phelps in Ann Arbor, Mich., also advanced to the evening semifinals. They were joined by 2004 Olympians Klete Keller and Larsen Jensen.

Phelps set the 200 free world record at last year's world championships in Australia, where his seven gold medals was the greatest performance in the meet's history.

Asked if he would lower the mark at the trials, Phelps said, "We'll see. Time will tell. I felt better this morning than I did yesterday, so it's a start."

Defending Olympic champion Aaron Peirsol qualified sixth-fastest for the evening's 100 backstroke semifinals. He finished second in his heat with a time of 54.14.

Randall Bal, fourth in the 2004 trials, was quickest in 53.28 — second-fastest in the world this year. Matt Grevers, also seeking his first Olympic berth, was second in 53.69, sixth-fastest in the world. Lochte was second to Grevers in their heat, and qualified fourth overall in 53.96.

Amanda Beard, seeking her fourth Olympic berth at 26, moved on in the 100 breaststroke, qualifying 11th for the semifinals.

American recordholder Jessica Hardy led the way in 1:06.85, followed by Rebecca Soni and Megan Jendrick, the 2000 Olympic champion who barely missed making the team four years ago. Also advancing was Tara Kirk, who was sixth in Athens.