Choi was 5 under through 12 holes when first-round play was delayed because of lightning, then added a birdie on the par-4 14th and finished the bogey-free round with four pars. The South Korean player won the U.S. Women's Open in July for her first major title and sixth LPGA Tour victory.
"I put extra pressure on myself, too, as a lot people expect me to win here, but I like it. It makes you more nervous and encourages you to play better," Choi said.
Webb also had a bogey-free round, joining Choi atop the leaderboard after the delay with a birdie on the par-3 17th. The Australian won consecutive events early last year in Singapore and Phoenix for the last of her 38 LPGA Tour victories.
"Six birdies and no bogeys, I'm pleased with that," Webb said. "It's always hard to stay in focus after you have to sit it out for more than three hours.
"Instead of wanting to finish it off quickly without making mistakes, I was trying to hit some quality shots and that made the difference."
Japan's Mika Miyazato, South Korea's Sun Young Yoo and 17-year-old Taiwanese amateur Min Lee were a stroke back at 66.
South Koreans Hee-Won Han and Hee Young Park shot 67, and Americans Cristie Kerr, Jessica Korda and Lizette Salas were another stroke back along with Japan's Ai Miyazato and Momoko Ueda, South Korea's So Yeon Ryu and Scotland's Cartriona Matthew.
American Stacy Lewis opened with a 70. In the last tour event, she won in Alabama on Sept. 23 to tie top-ranked Yani Tseng for the tour victory lead with three.
Michelle Wie had a 74. She had an eagle and three birdies, but lost eight shots with a triple bogey, a double bogey and three bogeys.
Tseng shot a 78, leaving her tied for 66th in the 71-player field. The Taiwanese star was 1 under after 10 holes, then dropped eight strokes in the next five with three double bogeys and two bogeys.
The tournament opens the tour's four-event Asian swing. The HanaBank Championship is next week in South Korea, followed by stops in Taiwan and Japan.