Miyazato, an LPGA Tour rookie, is one shot ahead of Il Mi Chung and Karrie Webb, who both shot 3-under 69 at a gusty Trump International Golf Club.
The field of 32 players will be cut to 16 after the second round, then to the top eight following three rounds.
Discarding the first 54 holes, the final standings will be determined only by a player's score in the final round. The winner receives $1 million of the $1.55 million purse.
She who plays best Sunday, wins.
'It's just really a grind,' said Natalie Gulbis, who had a 2-under 70. 'It's worth so much money, but you have to get through these first two days. It's going to be like playing a final round tomorrow.'
Making things more difficult Thursday was a swirling wind -- Wendy Ward called it 'unsettled' -- that gusted between 12 and 20 mph. Rain moved into the area shortly before 4 p.m. and caused a 31-minute delay.
The forecast for Friday looks calmer, with little chance for more rain.
'It was very windy today, but I tried to keep my concentration and I felt very good,' said Miyazato, who finished well before the rain delay.
Miyazato had six birdies to go along with a double-bogey at the par-4 10th, a hole that also produced a six for Annika Sorenstam.
The women's No. 1 -- a two-time defending champion and four-time ADT winner -- opened with a 2-over 74 and was among several top players who struggled Thursday.
In the first round last year, Sorenstam and Paula Creamer quarreled over where Sorenstam should a take drop at the 18th hole. Sorenstam won out, and a visibly irritated Creamer famously quipped, 'It's her conscience.'
This year, Sorenstam's biggest challenge in the first round was working out kinks in her swing. And it took a while.
She made bogey at the fourth and eighth holes, then joined playing partner Cristie Kerr with her double-bogey at the 10th. Sorenstam didn't make her first birdie until the 13th. A 12-foot birdie putt at the 16th put her at 2 over.
Money leader Lorena Ochoa, who has already locked up Player of the Year with six wins, also struggled in the first round, shooting a 3-over 75 to end tied for 22nd place. Ochoa missed a 10-foot putt at 16 for the last of her four bogeys.
Gulbis is tied for fourth place with Julieta Granada and Mi Hyun Kim at 2 under. Creamer shares seventh place with Morgan Pressel, Se Ri Pak and Wendy Ward at 1-under 71.
Creamer, who ended sixth last year, broke par with a 20-foot birdie putt at the 15th hole. It's the kind of shot, she figures, players will need to have in their bag this weekend.
'Your putter has to be working on Sunday,' she said.
Pressel held the lead at 3 under through 12, then bogeyed three of her next four holes. She rebounded with an 8-foot birdie putt at the 18th.