Miyazato, who won her ninth LPGA Tour title at the NW Arkansas Championship on Sunday, fired a two-under-par 70 in energy-sapping heat at Blackwolf Run to finish level with American teenager Lexi Thompson and Spaniard Beatriz Recari.
South Korean Lee Meena opened with a 70 while Swede Anna Nordqvist and five-times Major winner Pak Se-ri were among a group of six players knotted on 71 in the third Major championship of the year.
Taiwan's world number one Yani Tseng, who has claimed five of the last 10 Major titles, was among the day's late starters.
Miyazato played most of her round before the temperature peaked at around 92 degrees Fahrenheit (33 degrees Celsius) and said she had intentionally buried memories of her victory on Sunday.
"Last week was just last week," the 27-year-old world number three said. "I try to forget about my winning because this week is just a completely different golf course and different conditions, too.
"I just trying to shake off everything and make a fresh start, and that's why I think I played really well today."
Asked how difficult it had been to cope with the soaring temperatures and high humidity, Miyazato replied: "I'm lucky because I played in the morning, so it wasn't too bad.
"But the last five holes it was really hard. I had the ice pack in my golf bag ... and that helped. It wasn't easy to keep really strong focus. You are just trying to stay in the present, and not really think about this heat."
Thompson, one of the game's most exciting prospects, was delighted with a dogged opening round that included five birdies and three bogeys.
"It feels good," said the 17-year-old, who at 16 became the youngest player to win an LPGA title with her victory at the 2011 Navistar LPGA Classic.
"You just gotta stay patient out there any time at an Open.
On the back nine, I had a few bogeys and birdies, but the most important thing was that I bounced back, so I'm pretty positive about that."
Thompson, who at 12 became the youngest player ever to qualify for the US Women's Open, planned to adopt the same strategy of patience for Friday's second round.
"I feel really good about where I'm at right now, I'm just going to have the same attitude going into tomorrow," she said.
"I'm not going to play any differently ... just try to make pars and a few birdies when I can."
Tour veteran Pak, who won the US Women's Open when it was last held at Blackwolf Run back in 1998, was happy to flourish at the same venue 14 years later.
"It felt like a totally different golf course but I have such great memories about '98, so no matter what, I feel really great to be out there today," the 34-year-old smiled.
"I had a really solid round. I'm very happy about the way I'm playing. This golf course, you miss just one shot by a little and ... huge numbers all of a sudden show up."
South Korea's Ryu So-yeon, who won last year's US Women's Open in a play-off with compatriot Seo Hee-kyung, launched her title defence with a 74.