Missy Franklin combined irrepressible teenage ebullience with intense professional concentration on Friday to shatter the world record in the Olympic 200 metres backstroke.
Franklin, 17, took three-quarters of a second off the previous record, which had been set in a now-banned non-textile suit, with a time of two minutes, 04.06 seconds.
"She's an amazing competitor, she's always having fun but she really knows how to tune in when she needs to," said third-placed American team mate Elizabeth Beisel.
Beisel said Franklin's personality was contagious.
"In the waiting room I was pretty nervous and just watching her have so much fun really made me relax," she said.
Franklin was the first triple gold medallist of the London Games. She also won bronze in the 4x100 metres relay and a further gold seems certain in Saturday's medley relay.
After receiving her medal from Russia's Alexander Popov, the only male swimmer to win four individual Olympic freestyle gold medals, Franklin said she did not know that a world record awaited her at the end of Friday's race.
"I definitely took it out really fast," she said. "It hurt so bad in the last 25, that's the part that I love, knowing that I'm pushing myself past the limit. It means the world to me and I just wanted to go for my best time today."
Franklin showed her competitive steel at the U.S. trials where she flourished when the pressure was at its most intense to qualify for the 100 and 200 metres freestyle, the 100 and 200 backstroke and the three relays.
A medal haul of seven golds was always an unrealistic expectation and Franklin should return to Colorado with five, one fewer than team mate Natalie Coughlin took home from the 2008 Beijing Games.
Friday's world record was the seventh of the meeting, showing that times are now improving again after the performance-enhancing non-textile suits were banned in 2010.
"I think that you always have to think that what you want to achieve is possible," Franklin said.
"Just knowing that it is a larger mountain after those suits, an onward climb, a harder climb, it is going to be much more worth it at the end.
"And this pool is extremely fast, you're always hearing someone saying this pool is a fast pool. This pool has been my favourite by far.
"It means so, so much to me, every athlete dreams of having a world record."
Biesel said more people had been getting close to the world records set in the banned suits.
"Finally they are breaking them here. I think when one person breaks the barrier it really sets the stage for everybody else," she said.
She added that a new generation of American swimmers would come to the fore after the Olympic swimming programme finished on Saturday, headed by Michael Phelps, the most decorated athlete in Olympic history, who will retire after his final relay.
"There's so many people retiring, it's going to be a whole new group," she said.
Franklin said the goal for the United States was now to take the sport further forward.
"I think there are so many members of the team this year and the national youth team that are coming up that are going to help carry on this incredible generation that we had before us," she said.
"It is going to be incredibly hard because they have left unbelievable footprints in swimming history and just being able to follow that and to learn from them is going to help us all become better."