Feng holed six birdies at Trump National Golf Club, where inclement weather led to play being suspended for two hours during the afternoon.
The 27-year-old from China, winner of the 2012 PGA Championship, also won bronze at the Rio Olympics last year.
Continuing her bid for a first major title, Amy Yang carded a 67 to be outright second.
Behind Yang – who has finished in the top 10 at majors 16 times – is in a group of three, including star duo Lydia Ko and Ryu So-yeon.
World number one Ryu and the fourth-ranked Ko opened with 68s to be in a tie for third with Carlota Ciganda, who has a hole to play.
While Ryu went through bogey-free, Ko had an up and down round with six birdies and two bogeys.
A group of eight players are tied for sixth at three under, including 15-year-old amateur Rachel Heck and Nelly Korda – who have five holes to play.
Minjee Lee, Lee Jongeun, Choi Hye-jin, Kim Sei-young, Megan Khang and Cristie Kerr completed their three-under 69s.
Danielle Kang, who won the PGA Championship at the start of July, posted a one-over 73.
Defending champion Brittany Lang carded an even-par 72 in the first round.
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