South Korean Choi and Norwegian Pettersen, playing together, fired unblemished rounds of four-under-par 68 to top a crowded leaderboard at the opening women's Major of the year at the Mission Hills Country Club in Rancho Mirage, California.
World number 110 Ewart Shadoff needed six birdies to offset her two bogeys and stay with her more decorated rivals, a stroke clear of South Korean Amy Yang and Swede Anna Nordqvist at three-under.
World number eight Pettersen, winner of the 2007 LPGA Championship, has finished runner-up on three previous occasions at this event and was pleased with the start of her quest to go one better.
"Today was everything I could ask for opening round," Pettersen said.
"Just feeling really good all week, and it's just about kind of trusting what you have, and I couldn't ask for a better start.
"Bogey-free, gave myself a lot of chances, feeling good with my game, and if you hit a lot of fairways here you give yourself a lot of great looks on your approach shots, and I feel like I'm striking my irons good and I'm hitting a lot of shots that I'm seeing."
Reigning US Open champion Choi believed playing together had helped both players.
"I had a great round today," Choi said. "I played with Suzann and I think we motivated each other.
"I hit a lot of fairways and that's why I got a lot of chances for birdies and I feel great and I'm really looking forward to the rest of the week."
Ewart Shadoff's round featured four successive birdies from the 11th hole but the 25-year-old newlywed was not surprised at her good form.
"I've been playing really consistently the last three or four tournaments," she said.
"And so I knew, like my game is right there, and I knew I was due to have a really good round."
Americans Jane Park, Jacqui Concolino, Jessica Korda, Lizette Salas and Angela Stanford joined Koreans Shin Ji-yai, Park Hee-young and Inbee Park plus German Caroline Masson, Italian Giulia Sergas and Thailand's Moriya Jutanugarn in an 11-way tie for sixth just two off the lead.
World number one American Stacy Lewis failed to produce her best on the way to a one-over 73 to be tied for 44th, while defending champion Yoo Sun-young of Korea struggled to a five-over 77.
Top ranked amateur, New Zealand's 15-year-old Lydia Ko, opened up with an even par 72.
- Sports & Recreation
- Mission Hills Country Club