Japan's Ai Miyazato, winner of last weekend's LPGA Tour event in Arkansas, was a shot off the pace along with 17-year-old Lexi Thompson, her fellow American Jennie Lee and Spain's Beatriz Recari at Blackwolf Run in Kohler, Wisconsin.
World number one Yani Tseng - aiming to become the youngest player to complete a career Grand Slam - was in a large group five shots adrift of the leading trio along with defending champion So Yeon Ryu after starting with a 74.
Cheyenne Woods, the niece of 14-major winner Tiger and making her debut in her national championship after turning professional earlier this season, opened with a 75 in the energy-sapping heat.
The 21-year-old qualifier said: "I didn't start off too good. I was three over through nine and I went birdie, birdie. I felt like I held in there pretty good. I bogeyed my 16th and 17th hole, but I feel pretty good about my round overall. It's a tough course, and I'm just happy to be here."
Asked if she expected to hear from her uncle, she replied: "Would I? Yeah. He's always been supportive of my career and he's excited that I'm taking this next step."