Miyazato, the 2010 winner, won last year at the Evian Masters. She has seven players within two strokes and 16 players within three shots of her lead.
Christel Boeljon and Numa Gulyanamitta share seventh place on the Pattaya Old Course at Siam Country Club at minus-three.
Women's world No. 1 and defending champion Yani Tseng opened with a one-over 73 and is tied for 36th.
Miyazato, a seven-time winner on the LPGA Tour, opened with a birdie on the par-five first. She cruised to seven consecutive pars from the second.
The 26-year-old rolled in a birdie try at the ninth to make the turn at two- under. She converted her next birdie effort at No. 12.
Miyazato got to four-under as she birdied the 14th. She posted her final birdie on the par-three 16th. Miyazato parred the last two holes to close out a bogey-free round.
"Overall, I hit the ball well and my putting was really good," Miyazato said. "I had a few really good up and downs. So I could keep the round going today so I was very happy about that."
Webb parred all nine holes on the front nine. Around the turn, she birdied three of the first four holes. Webb's final birdie was at the 16th.
Nordqvist had a bogey-free round with a birdie at the second and three more birdies on the back nine.
"I finished strong in Australia last weekend. So I had some confidence from that," said Nordqvist. "So I just hit the ball really smooth today so I gave myself a lot of birdie opportunities today. I never really had to fight for par, which I think is key out there and I was also able to get a few birdie putts to roll in."
Pak had three birdies and a bogey on the front nine. She birdied the 14th and 18th to grab a piece of second place.
Yang birdied the fourth, but gave that shot right back with a bogey on the fifth. She atoned for that mistake with a birdie at the seventh, then had three birdies on the back nine to end at minus-four.
Choi had eight pars and a single bogey on the front nine. She caught fire around the turn with four birdies in the first six holes. Choi also birdied the last to grab a piece of second.
NOTES: Paula Creamer had her first career hole-in-one on No. 12, but only managed to shoot even-par 72...Four of the five previous winners are in the field with just the retired Lorena Ochoa not competing...Suzann Pettersen, the 2007 winner, is one of nine players tied for ninth at minus-two, while 2006 champion Hee-Won Han struggled to a three-over 75 and is tied for 55th.