Dana Vollmer set a new world record as she won gold for the United States in the women's 100m butterfly.
The 24-year-old world champion won in 55.98 seconds, with China's Ying Lu earning the silver medal, edging out Australia's Alicia Coutts in third.
Great Britain's Ellen Gandy finished eighth but will hope to fare better in the 200m butterfly.
"I would like to have been faster but it was an amazing swim by Vollmer and nice to be part of that," Gandy said.
"My strength is in the 200m and I want to get stuck into that now."
Vollmer, who won Olympic gold as a 16-year-old in 2004 but failed to make the US team for the Beijing Olympics, beat Sweden's Sarah Sjostrom's three-year-old world best by 0.08 seconds.
"I felt like I had that time in my mind and I kept being so close," Vollmer told BBC Sport. "I felt so excited and so good, it was awesome to look up and see that I got it."
It is her country's second gold medal in the pool after Ryan Lochte's win in the men's 400m individual medley on Saturday.