Reid carded a brilliant four-under-par 67 on the notoriously tough Lake Course at the Olympic Club in San Francisco.

The English player started at the 9th hole and hit her first two approach shots within 10 feet for birdies on the par-4s.

She added further birdies on 15 and 16, another on her second-to-last hole and had only one bogey all round.

"I didn't think that score was out there honestly," Reid said.

"I had a pretty good game plan. It's probably the best I've had for a tournament. We had a game plan and stuck to it. If you're in trouble, just get it out, make bogey.

"The key here is to not take many risks the first two, three days, and I didn't do that."

"He (Brooks Koepka) gave me a few things that he follows by in a major, so obviously appreciate his help. What he told me was, I thought, invaluable honestly." – Mel Reid

Of the 78 players who teed off in the morning, only eight shot under par.

Angel Yin was a stroke off the lead, Yuka Saso shot 69, and top-ranked Jin Young Ko was at 70 with Marina Alex, Austin Ernst, Jennifer Kupcho and Jeongeun Lee6.

Reid, who 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.

Green and Lee both carded two-over 74s to be six shots off the lead in a tie for 56th.