Taking advantage of some favourable tee positions, the world number one birdied four of her last nine holes to fire a superb five-under-par 67 at Sebonack Country Club, ending the day a stroke behind her pacesetting compatriot Kim Ha-neul.
Another Korean, Kim In-kyung, was also in early contention for the season's third Major after carding a 68 to finish level with American Lizette Salas and Swedes Caroline Hedwall and Anna Nordqvist.
However, Park will have given every other player in the field plenty to think about after surging to the top of the leaderboard in her bid for a sixth victory on the 2013 LPGA Tour, and the fourth Major title of her career.
"I played very good today," the 24-year-old Korean said after mixing six birdies with a lone bogey and totaling only 25 putts. "I hit the ball very good, didn't miss many fairways or greens."
Tournament organisers had moved several of the tees up due to concerns over a poor weather forecast for Thursday, and Park took advantage with some aggressive approach play.
"I was able to attack some pins where the USGA (United States Golf Association) was a little generous on us, a lot of tees were moved up," she said.
"So instead of hitting like five-irons, we were hitting nine-irons, and that made the course much easier.
"I was actually able to go for some pins and give myself a lot of opportunities today. I made a lot of putts and didn't leave much out there."
Having already won this year's Kraft Nabisco Championship and LPGA Championship, Park is oozing self-belief as she seeks to emulate Babe Zaharias (1950), Mickey Wright (1961) and Pat Bradley (1986) by clinching three Major titles in one season.
"I do have a lot of confidence in myself at the moment ... the way I'm playing, the way things have been going, the way I've been getting the luck," said Park.
"I think I am in the zone. I've been playing my best in my career at the moment. I really just want to enjoy the moment."
Park, who at 19 became the youngest ever winner of the US Women's Open with a four-shot victory in the 2008 edition at Interlachen, was a heavy favourite coming into this week following wins in her previous two starts on the LPGA Tour.
"I've played very good golf the last two or three months," the South Korean said. "Everything's been going the right way. Everything's going the way I really want it to."
Park held at least a share of the lead for most of the day until Kim birdied her 17th hole, the par-five eighth, to edge one shot clear.
"This is my first time in US Open, and I didn't think that I'm going to do it like this," the 24-year-old Kim said through a translator after piling up six birdies in a bogey-free display.
Kim has triumphed seven times on her home LPGA of Korea Tour where she led the money list in 2011 and 2012.
Paz Echeverria of Chile and Canada's Maude-Aimee Leblanc opened with 69s at Sebonack while defending champion Choi Na-yeon of South Korea and American world number two Stacy Lewis were among a group of 11 players knotted on 71.
However, several big names struggled in the opening round, third-ranked Norwegian Suzann Pettersen battling to a 76 and American Michelle Wie faring even worse as she laboured to an eight-over 80.
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