American Allison Schmitt, a training partner of Michael Phelps, swam an Olympic record to win the gold medal in the women's 200m freestyle on Tuesday.
Schmitt went one better than her silver medal in the 400m freestyle earlier in the Games to score a clear-cut victory in one minute 53.61 seconds.
The 22-year-old finished almost two seconds clear of French 400m Olympic gold medallist Camille Muffat (1:55.58) and Australian Bronte Barratt (1:55.81).
"I was just racing as hard as I could. I knew I had to kick it in such a strong field," Schmitt said.
"When I looked up and saw 53 (time) on the scoreboard and first place next to my name I couldn't have been happier. I can't put into words how I feel right now, I am speechless."
Phelps and his coach Bob Bowman enjoy having the company of fun-loving Schmitt in their training set-up at Baltimore.
"We have had a great training group and I think that was one of the reasons is really having Allison Schmitt around," Bowman said at the US Olympic trials.
"Having her there was wonderful, kind of lightened things up a little bit. And she is a jokester, and that's something cool we have in the pool now."
American teen star Missy Franklin, who won her first individual Olympic gold medal in the 100m backstroke final on Monday, missed out on a medal on Tuesday by just one-hundredth of a second.
Franklin was bidding to win a medal in each of her four individual events and three relays in London, a programme that would make her the first US woman to swim seven events at one Olympics.
"I am just trying to do the best I can, I knew it was going to be an incredible race and I just wanted to do my best time," Franklin said.
"It's hard when it comes down to such a close time."
Muffat, who is savouring her 400m freestyle triumph, said it was all catching up with her.
"I am really tired now and we have the relay left, after that I will be able to enjoy the competition from the other side," she said.
Italy's defending champion Federica Pellegrini was a well-beaten fifth.