The Australian pair both carded eagle threes on their way to four-under 68s, joining eight other players in a tie for 11th.
World No.18 Alison Lee shot a seven-under 65 to top the leaderboard after the first round.
Lee had a busy opening round that featured an eagle, seven birdies and two bogeys at Blythefield Country Club, giving her a two-shot lead over a group of nine golfers tied for second at 67.
After an outward nine featuring a bogey, two birdies, and an eagle on the par-5 8th hole, Sydneysider Kim dropped one more shot before finishing with a flourish with successive birdies on the 17th and 18th.
Starting her round on the 10th hole, the 24-year-old Ruffels enjoyed a similar surge, gaining four shots between the 13th and 15th as she carded birdie-eagle-birdie.
"There can be some days I don't feel quite confident. You just have to work with what you got. That's how I felt today.” – Alison Lee.
Perth's Hira Naveed (69) is a shot further back at three-under, while two-time major winner Minjee Lee (72) and Sydneysider Robyn Choi (73) look to be all but out of the reckoning.
Alison Lee, a 29-year-old from Los Angeles, has yet to win on the LPGA Tour and battled with her swing early in the round.
"Confidence is a huge factor," Lee said.
"There can be some days I don't feel quite confident. You just have to work with what you got. That's how I felt today.
Gabi Ruffels is right in the mix in Belmont. PHOTO: Getty Images.
"As the round went on, I would say the last four, five holes, definitely felt a lot better with my swing."
Canada's Brooke Henderson, who won this tournament in 2017 and 2019, eagled the 8th and 18th holes to account for most of her scoring as she finished two shots back in joint second.
Of the group of eight players tied with her at 67, the only ones to stay bogey-free were Lizette Salas, from the U.S, and Korea's Narin An.
Defending champion Leona Maguire of Ireland opened with an even-par 72 while World No.1 Nelly Korda shot a four-over 76 in her first event since missing the cut in the U.S Women's Open.
