"Other than messing up my hair, it was fine," she quipped after jogging the last 100 metres in a steady rain to win her heat.
The Jamaican-born American, who is bidding for spots on the American team in the 200 and 400 metres, ran hard the first half of her race before easing off to finish in 51.69 seconds.
Olympic champion LaShawn Merritt topped the men's preliminaries in 45.36 seconds. Former Games gold medallist Jeremy Wariner was 13th fastest of 16 qualifiers. He ran 45.84.
The big early marks belonged to decathlon world silver medallist Ashton Eaton.
The hometown favorite, accustomed to the rain in his native Oregon, splashed to the fastest 100 metres ever run in a decathlon, 10.21 seconds, to start the competition.
"Ashton Eaton is a true beast," tweeted 2004 Olympic 100m champion Justin Gatlin.
But Eaton was not finished, later leaping 8.23 metres, an unprecedented mark in the long jump leg of decathlon competition.
His third event was more subdued as he finished fifth in the shot put, one of his weaker events.
Still, the indoor heptathlon world record holder streaked to 2,905 points with an overwhelming 149-point lead over world champion Trey Hardee.
Hardee, the shot put winner, was on 2,756 points, while Olympic champion Bryan Clay was third with 2,725.
The top three finishers in their events at the US trials, which continue to July 1, book their tickets to London provided they meet the Olympic qualifying standard.