The South Korean has moved on to eight-under-par overall and is ahead of compatriot Mirim Lee, who fired a record-equalling round of 64 on the opening day yesterday.

Amy Yang, another South Korean, is level with Lee on six-under.

Park's round included seven birdies and came on her US Open debut.

She was able to leap-frog Lee who went round with a two-over-par 74, ten shots worse than her efforts yesterday.

"Coming to the tournament, I didn't even think about winning because this is the first time for me," said Park. 

"I'm trying to enjoy this tournament. 

"That's why I am comfortable, I don't even think about the winning."

The Koreans will be looking ominously over their shoulders, however, as world number one Lydia Ko now lurks behind the trio on five-under-par.

The 19-year-old, born in South Korean capital Seoul but who now represents New Zealand, enjoyed a round of 66 to improve on her 73 yesterday.

It means she is poised to pounce as she searches for a third major title.

Canada's world number two Brooke Henderson, the winner of the last major, the PGA Championship last month, is well adrift on three-over-par but did make the cut.

"Just to know that this is the first time I was under par for the tournament kind of put myself in a positive position," said Ko. 

"I just tried to enjoy it out there."

The third round is due be played tomorrow.