Thompson carded eight birdies and one bogey to seize the early initiative before Ji surged into contention later in the day with a bogey-free round in Mobile, Alabama.
Jessica Korda of the United States was a shot back in third after signing for a 66, with seven players a further stroke adrift on five under - American trio Nicole Castrale, Mina Harigae and Jennifer Johnson, South Korean pair Chella Choi and Hee Young Park, Holland's Dewi Claire Schreefel and Thailand's Thidapa Suwannapura.
"I love the par fives here, I get to hit pretty much driver on every hole out here so it's definitely an advantage for me," Thompson, who started her round on the 10th hole, said on www.lpga.com.
The 18-year-old, who birdied 16, 17, 18 and then recovered from a bogey at the second by stringing together another three straight birdies from the fifth, added: "I definitely had some momentum.
"I made a few birdies at the end of my first nine. I was hitting it pretty close and I drained a few putts.
"So I was just taking one shot at a time, not trying to get ahead of myself because it's golf, anything can happen, and I was lucky enough to make a few more birdies on the second nine."