The 17-year-old from New Jersey made back-to-back birdies on the back nine to take sole possession of the lead before making bogey on the 18th hole to end the day in a tie with Mel Reid on the Lake Course at the Olympic Club.

Ganne became the first amateur to lead after a round at the women's Open since Jane Park did it after one round at Newport Country Club in 2006.

"I think just my ability to play smart and not take any unnecessary risks, and I didn't panic when I got into the rough a couple of times out there," Ganne said of the key to her success.

"Because there are definitely holes I wasn't keeping in the fairway, and it's easy to panic out there, and I didn't do that."

Ganne needed a playoff last month to qualify for her second US Open but felt much more comfortable once she got here than she did two years ago when she missed the cut.

"The first time is nerve-racking for anybody and meeting your idols and being on the stage for the first time," she said. "But the second time around, even the practice rounds, I wasn't as nervous. I felt like I could come here and just play my game instead of soaking that all in."

The notoriously tough Lake Course played a little easier than usual after the rough was trimmed a bit before the round.

Fifteen players shot under par with Canada's Brooke Henderson, and Americans Angel Yin and Megan Khang one shot back.

"There are definitely holes I wasn't keeping in the fairway, and it's easy to panic out there, and I didn't do that."  Megha Ganne

Lexi Thompson, Yuka Saso and Shanshan Feng were two shots back.

Other notable players include defending champion A Lim Kim of South Korea, who struggled at six over, and 2014 champion Michelle Wie West, who shot 74.

Reid defied the tough going to claim the early clubhouse lead as Aussie big guns Hannah Green and Minjee Lee struggled to keep pace.

Green and Lee both carded two-over 74s to be six shots off the lead in a tie for 41st.

Fellow Australians Emily Mahar (77), Sarah Kemp (78) and Sarah Jane Smith (78) have plenty of ground to make up to make the halfway cut.

Reid, who only won her first LPGA Tour title last October to go with her six career wins on the European Tour, has had little career success at the US Women's Open.

She missed the cut four of her previous five times at this tournament and finished tied for 50th in her other appearance in 2012.

Reid said she was helped by a couple of long conversations with two-time men's US Open champion Brooks Koepka that she carried over into the round.

"He gave me a few things that he follows by in a major, so obviously appreciate his help," she said. "What he told me was, I thought, invaluable honestly."

This marked the first time the women came to the Lake Course overlooking the Pacific Ocean for a major. But this venue has a rich history for the men, hosting five US Opens and three US Amateurs among other events.