Karis Davidson and Minjee Lee shot two-under-par 69s on Thursday (Friday AEST) at Riviera Country Club to be in a six-way tie for eighth.
Compatriots Hannah Green and Stephanie Kyriacou are a further stroke back, Grace Kim (72) is one over and Sarah Hammett (80) nine over.
Lee's bogey-free first round of the second women's major of the year was highlighted by birdies on the first and ninth holes.
The 156-player field are fighting for the $US12 million ($A17 million) in prize money and American Jennifer Kupcho leads the way, her five-under 66 earning a one-stroke lead over South Korea's Sei Young Kim.
The 29-year-old Kupcho, who has four career LPGA Tour wins but just one since 2022, made seven birdies and two bogeys.
"I just really, really like the golf course," said Kupcho, who missed the cut in her three previous US Open starts.
"It's kind of a ball-striker's paradise. Just hit it down the fairway, hit it on the green and make the putts."
The entire field managed just one eagle in the first round - Allisen Corpuz holed out with a six-iron from 86 yards on the 17th.
Mexico's Gaby Lopez and Japan's Hinako Shibuno joined South Korea's Hyunjo Yoo, Ina Yoon and Minji Kang in the group at 68.
World No.1 Nelly Korda (73) made a rough start, struggling off the tee to finish two over.
"It wasn't a great day," Korda said.
"I hit it really good Monday through Wednesday, so I have honestly no idea where this came from. So I'm going to go to the range."
Aphrodite Deng, a 16-year-old amateur, was in contention for the lead before playing two late holes at three over and carding a 70.
Deng was born in Canada to Chinese parents before growing up in New Jersey and eventually settling in Florida.
A competitive child figure skater for several years before the COVID-19 pandemic, Deng took up golf only seven years ago, but won the US Girls' Junior last year.
- with the AP
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