Australia's Libby Lenton became the first woman in history to break 53 seconds for the 100m freestyle when she swam 52.99 during the first leg of the mixed relay at the Duel in the Pool swim meet against the USA here Tuesday.

Lenton's time was 0.31 faster than the 53.30 recorded by Germany's Britta Steffen when she won the European Championships in August 2006.

But because of the unusual nature of the race, the Australian will have an anxious wait to see whether her world record is ratified by the sport's governing body Fl NA.

The result was not enough to clinch the match for Australia, as the USA won the meet for the third time in

succession, scoring 181.5 points to Australia 129.5.

Lenton swam the first leg of the relay against multiple world champion Michael Phelps and there was initially some concern from swimming officials that Fl NA may decide that Phelps was pacing the Australian.

However, Swimming Australia spokesman David Lyall said later there was no reason to believe the record would notstand.

"We've spoken to Fl NA and they said they expected the record to be ratified," Lyall said.

"We won't know for certain tonight but we are confident it will stand as a record," he said.

It will be a double celebration for Lenton if she is awarded the world record as she is due to marry fellow swimmer Luke Trickett on Saturday.

"I think the wedding is going to top everything," she said.

"I don't think you can really compare the two, it is a very special personal momentforme."

Phelps, who won a record seven gold medals at the World Championships, swam only two individual events at the Duel in the Pool but he won them both comfortably.

He clocked 48.79 to win the 100m freestyle ahead of Australia's Eamon Sullivan (49.37) and fellow American Cullen Jones (49.76), then beat compatriot Aaron Peirsol and Australia's Ephraim Hannant to win the 200m backstroke in a time of 1:56.29.

In the major upset of the meet, Tara Kirk of the US beat Australia's world record holder Leisel Jones in the 100m breaststroke.

Kirk went out fast early on and Jones just failed to reel her in, the American touching in 1:06.72 ahead of Jones in 1:06.80.