The LPGA Tour's rookie-of-the-year leader is just two shots behind South Korean leader So Mi Lee, who piled up eight birdies in a sizzling six-under 66 on Thursday (Friday AEST).
Ruffels, who finished third at last week's Cognizant Founders Cup also in New Jersey before successfully qualifying for the US Open on Monday, again upstaged some of women's golf's biggest names.
Dual major-winning compatriot Minjee Lee is one stroke behind in an 11-way tie for sixth, with world No.1 Nelly Korda a further shot back in a share of 17th after opening with a two-under 70.
Ruffels was the only player four under or better to go bogey-free. She birdied three of her first eight holes before adding one on the back nine.
The 24-year-old feels she's "definitely getting more comfortable" as her rookie season wears on.
"I feel like the confidence definitely goes up when you have a few good results and I've been fortunate to have a few good results this year, few top-fives and one last week," Ruffels said.
"But still kind of feel like a rookie. Kind of learning all the new courses. These girls have already seen they courses, but I'm lucky I have a pretty experienced caddie."
Leader Lee, a first-year rookie like Ruffels, was the second alternate in the field and not expecting to get the green light to compete this week.
"At first I was kind of depressed and I think I can't play well today," the 25-year-old said.
"I also get (my) caddie to go home and we have no caddie, so my father play with me and that made me comfortable playing. Yeah, he was very great."
Leader So Mi Lee lines up a shot during her first round in New Jersey. (AP PHOTO)
Lee started her day on the back nine at Liberty National Golf Club and opened with a birdie at the par-5 10th. She made her move with a stretch of four birdies and one bogey from holes 13 to 17 before adding a bogey and three birdies on the front nine.
"At first it was so windy when I was playing first, but second shot was very great," she said.
"My second shot was very great, and ... the putting was getting along with my shot."
Joining Ruffels at four under in equal second were Bianca Pagdanganan of the Philippines, Japan's Mao Saigo and Andrea Lee.
Major winners Jennifer Kupcho, Thailand's Patty Tavatanakit, Celine Boutier of France and South Africa's reigning Australian Open champion Ashleigh Buhai are part of the large tie at three under with Minjee Lee.
Rose Zhang, the 20-year-old phenom who won her professional debut at last year's Mizuho Americas Open and also won last week, withdrew pre-round due to illness.
With AAP.
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