The two Swedes grew up playing together.
"Certainly seeing her shooting 4 under in the morning session gave me a little bit of inspiration for the afternoon," Nordqvist said.
Salas, a 23-year-old former Southern California star, played with Park in the last group of the final round of this year's Kraft Nabisco Championship. Three strokes back starting the day, she opened with a double bogey and tumbled to 25th after shooting a 79.
She bounced back to reach a playoff at the LPGA Lotte Championship in April, losing to Suzann Pettersen for her best finish on tour.
"I'm just getting a lot more used to being in contention and really studying the leaderboard and really managing my patience," Salas said. "I think that's been key for me this week. Yes, I still get nervous on the first tee and my hands keep shaking, but I just know that if I just trust myself and trust my instincts, I can perform out here."
Chile's Paz Echeverria, a 28-year-old LPGA Tour rookie also making her U.S. Women's Open debut, and Canada's Maude-Aimee Leblanc shot 69.
Among eight players at 70 was Natalie Gulbis, who withdrew from a tournament and missed two others earlier this year because of malaria. Infected by a mosquito during the LPGA Thailand in late February, she returned for the Kraft Nabisco in early April. Gulbis hasn't finished better than 13th since, missing the cut at the LPGA Championship.
Defending champion Na Yeon Choi, second-ranked Stacy Lewis and amateurs Kyung Kim and Brooke Henderson were among 11 players at 71.
Lydia Ko, the 16-year-old New Zealand amateur who won the Canadian Open last August to become the youngest LPGA Tour winner, had a 72. Juli Inkster, playing in a record-breaking 34th U.S. Women's Open at age 53, holed a 103-yard wedge shot for eagle on the 18th to also finish at 72.
Michelle Wie opened her round with a quadruple-bogey 8 on No. 10. She was at 11 over through 14 holes before birdies on three of the last four to finish with an 80.
With Park's two major titles to start the year, South Koreans have won four straight majors. But Ha-Neul Kim was an unlikely representative to lead after the first round of this tournament.
"I was very nervous coming in, and I thought in the practice round that the course was very difficult," she said. "Before playing today I thought that even par would be a very good score for me."