Four shots behind Thompson at 134 were New Zealand's Lydia Ko, the world number one, who carded a 67, and American Candie Kung, whose 66 in the second round included an eagle on the par-4 sixth hole.
Five back are Chung In-gee (66) of South Korea and Vicky Hurst (67).
Thompson and the rest of the field endured a rain delay, but the long-hitting American was stellar with six birdies and no bogeys while hitting 16 of 18 greens in regulation on the River Course at Kingsmill Resort.
"It's just a good golf course for me," she said. "I get to hit a lot of drivers, actually, a lot of 3-woods out here as well. I just love the shape of the golf course. It's in perfect shape for us once again this year."
Thompson is playing in her third event since she lost the first major of the year, the ANA Inspiration, in a playoff after a television viewer called in a rules violation and she was assessed a penalty.
"I feel great with where my game is at," she said.
Piller, who was in a group that trailed Thompson by one shot after the opening round, had another solid round but alternated birdies and bogeys on the final four holes of the day.
"Made some putts early and just couldn't get up and down the last couple," said Piller, who has never won on the LPGA Tour.
Ko was in good position after back-to-back 67s. She was bogey-free in the second round as she pursues her first win since July.
"All I need to do is focus on my game and be excited for the weekend," she said.
Defending champion Ariya Jutanugarn of Thailand, who is challenging Ko for the No. 1 ranking, was nine strokes behind the leader after a 67.
(Editing by Larry Fine)
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