"My feeling today was so awesome," the long-hitting Park said. "It was a good start after the major championship winning ... I need to focus more on the Marathon Classic and want to keep my happiness behind from now on."
Park is trying to become the first person to win the U.S. Women's Open and the following event since Meg Mallon in 2004.
Piller, whose best finish of 2017 was a solo fourth at the Pure Silk Bahamas LPGA Classic, made nine birdies and just one bogey Thursday
"I birdied my first three holes and any time you can start off three in a row, it's pretty good," Piller said.
The 32-year-old American, who has never won on the LPGA Tour, started her day with four birdies in her first five holes.
Chien's first-round 64 is the lowest round of her LPGA career, besting the second-round 66 she shot at the 2017 Kia Classic.
Aditi Ashok of India, Kim In-kyung of South Korea, and Kelly Shon were two shots back.
In the opening round of her title defense, former world number one Lydia Ko of New Zealand registered two successive bogeys on the back nine to finish with a 73, 10 shots off the lead.
(Editing by Larry Fine)
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