Australia's Cate Campbell set a US Open record of 53.30sec in winning the women's 100m freestyle at the Santa Clara International Grand Prix swimming meet.
Natalie Coughlin was second, improving her own American record with a time of 53.39.
Campbell's time is equal to the former world record set by German Britta Steffen in 2006 and has only been bettered by countrywoman Libby Trickett, who holds the world record at 52.88.
Coach Simon Cusack said he was extremely pleased with his young charge: "Going into the final tonight I thought she could be capable of going a sub-53.50 coming off some of the work I have seen in the last couple of weeks in training and she put it all together and did a great job and I am really happy with it."
Campbell, 15, also led off an Australia squad that captured the women's 4x100m free relay with a time of 3:43.08.
There was a strong showing for Australia's women in the 400m free as well, with Bronte Barratt and Kylie Palmer finishing one-two. Barratt took home gold with an Australian and meet record of 4:04.57. Palmer was second in a personal best 4:05.98 ahead of American Kate Ziegler (4:07.11).
Barratt's time ranks fifth in the world in 2008 but, considering she is still in heavy training, it was a surprise to the 19-year-old.
"A nice surprise because I have been feeling pretty bad in the water and we are still in full training," Barratt said.
"At the (Olympic) trials I tried to go out too hard and I had nothing left when I needed it so we have been trying to work on my backend, which has been my weakness, so its obviously starting to pay off."
A total of four meet records fell during the first full day of competition.
American Brendan Hansen won gold in the men's 100m breast with a meet-record time of 1:00.01. Slovenia's Damir Dugonjic was second in 1:01.67 and Japan's Ryo Tateishi was a close third in 1:01.68.
Superstar Michael Phelps, preparing for a record-breaking bid at the Beijing Olympics in August, took home his first gold of the meet with a win in the men's 400m individual medley.
Phelps led throughout the race and finished with a time of 4:13.47. Robert Margalis was second in 4:15.05.
Japan's Izumi Kato won the women's 200m breaststroke in 2:27.69, ahead of former world record-holder and reigning Olympic champion Amanda Beard (2:28.15).
The men's 4x100m free relay saw a strong South African squad of Lyndon Ferns, Roland Schoeman, Gideon Louw and Ryk Neethling win in 3:15.58.
The meet continues Saturday with 10 events.
