Kim earned a spot in the tournament by finishing first on the Korean Tour's money list last season and she took advantage of the opportunity to shoot a 6-under-par 66 at the Sebonack Golf Club.
That put her one shot in front of world No. 1 Inbee Park, who has five victories on the LPGA Tour this season with two of them being in major championships.
Park is attempting to become the second player in tour history to win three major titles in the same year. Babe Zaharias has been the only person to do so, having pulled off the feat 63 years ago.
The opening round was completed under cloudy skies on the eastern tip of Long Island, but the players dodged heavy rains which fell nearby.
Kim went out in the next-to-last group of the afternoon and jumped to the top with birdies on three of her final six holes. Park turned in her 67 early in the day, getting off to a hot start with three birdies in her opening four holes.
Those tied at 4-under 68 included Caroline Hedwall, Lizette Salas, I.K. Kim and Anna Nordqvist. I.K. Kim has finished in the Top 10 of this tournament four of the last five years and is best remembered for missing a short putt on the final green that cost her a win at the Kraft Nabisco Championship last season.
There were 27 sub-par rounds recorded on opening day after USGA officials shortened the course in anticipation of poor weather conditions. Gusty winds were in the forecast for Friday's second round.