The 25-year-old Australian shot a five-under-par 67 to finish four behind runaway leader Chanettee Wannasaen at soggy TPC River's Bend.
Ruffels was travelling well with four birdies on the front nine before finding trouble with a double on the par-four ninth.
But she steadied immediately with back-to-back birdies on 10 and 11, before adding another on the 15th to move into a four-way tie for ninth, that includes world No.2 Nelly Korda among others.
Wannasaen holed out a nine-iron for eagle on the par-4 10th and had seven birdies in her morning round.
"Actually, today play really good," Wannasaen said. "After AIG (Women's British Open), I have a little bit struggle with my swing, so like I cannot hit like a solid shot."
The 21-year-old Thai player has two LPGA Tour victories, the 2023 Portland Classic and 2024 Dana Open.
American Gigi Stoll was second at 65. She played the four par-5 holes in five under with an eagle on the 11th and three birdies.
"I think they're a little shorter than usual, so playing the par 5s well out here is going to be a big advantage," Stoll said. "I just got the ball rolling today."
Sei Young Kim was another stroke back with Patty Tavatanakit, A Lim Kim, Peiyun Chien, Kumkang Park and Jenny Bae.
Korda is searching for her first LPGA title this year after winning seven times in 2024.
"Made a couple putts here and there," Korda said. "Overall, just kind of everything was flowing and the weather was great.
"There was really no wind. So with kind of the saturated conditions, a little softer, you could be a little bit more aggressive."
Charley Hull opened with a 68. She's coming off a runner-up finish in Houston as part of the Aramco series on the Ladies European Tour.
"I played pretty solid last week and, obviously, now first week back on the LPGA carrying on into this week," Hull said. "Hopefully, we can go low over the next few days."
Top-ranked Jeeno Thitikul shot 69, along with Australians Grace Kim, Steph Kyriacou and world No.4 Minjee Lee.
Kim, who won the Evian Championship in July for her first major, began her round - starting on the back nine - with three straight birdies.
The world No.28 made another birdie on the 18th, a bogey on the third her only blemish on the day.
Kyriacou had five birdies and two bogeys, while Lee had four birdies and a bogey.
Perth's Hira Naveed is one shot further back along with defending champion Lydia Ko.
World No.17 Hannah Green had a three-over 75.
with AP
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