The 18-year-old Thompson birdied four of her last five holes for a seven-under 65 on The Crossings course at the Robert Trent Jones Golf Trail's Magnolia Grove complex.
Ji, the 2009 US Open champion, had five birdies in a six-hole stretch on the front nine in her bogey-free round.
Jessica Korda was a stroke back with Nicole Castrale, Mina Harigae, Dewi Claire Schreefel, Thidapa Suwanapura, Hee Young Park, Jennifer Johnson and Chella Choi tied for fourth after 67s.
"I was just trying to go out and make some birdies," 2011 winner Thompson said. "I'm glad to be back in Alabama."
World number two Stacey Lewis, a winner this year in Singapore and Phoenix, opened with a 70, while Michelle Wie shot a 74.
- Sports & Recreation
- Robert Trent Jones Golf Trail
- Hee Young Park
- Nicole Castrale
- Mina Harigae
- Jessica Korda
- Dewi Claire Schreefel