Natalie Coughlin waves to the crowd following her win and new world record during her heat in the women's 100m backstroke at the US Olympic Swimming Trials in Omaha, Nebraska, June 30, 2008. [Agencies]

OMAHA, Neb. - Natalie Coughlin reclaimed her world mark in the 100-meter backstroke one heat after Hayley McGregory broke the mark at the US Olympic swimming trials on Monday.

Coughlin won her preliminary heat in 59.03 seconds Monday.

One heat earlier, McGregory finished first in 59.15, bettering Coughlin's mark of 59.21 set February 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."

Hayley McGregory celebrates her new mark of 59.15, which was later broken by Natalie Coughlin during the women's 100m backstroke heat at the US Olympic Swimming Trials in Omaha, Nebraska, June 30, 2008. [Xinhua]

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 about 2 minutes before Coughlin reasserted her dominance as the defending Olympic champion.

Their efforts made Coughlin and McGregory the two leading qualifiers for the evening semifinals, which involve 16 swimmers.

"I was planning on going a lot easier this morning, but it gave me motivation to swim a little faster than I was originally planning," Coughlin said. "Just felt like a nice strong swim. I'm happy with that."

McGregory just missed qualifying for the 2004 Olympics when she finished third in the 100 back at trials. Only the top two in the final earn berths.