The 18-year-old Thompson, second last year behind Stacy Lewis, had eight birdies and a bogey on The Crossings course at the Robert Trent Jones Golf Trail's Magnolia Grove complex.
''I definitely had it in mind, but it's a whole different year and a new day, so I was just trying to go out and make some birdies,'' Thompson said. ''I knew my game was good, so just going to try to keep that going.''
She hit to inches on the par-4 ninth to set up her closing birdie.
''That was a good shot to end on,'' Thompson said. ''I just had a three-quarter pitching wedge and just committed to it. ... I was hitting it pretty close and I drained a few putts. I was just taking one shot at a time, not trying to get ahead of myself because it's golf, anything can happen.''
She won the 2011 Navistar LPGA Classic in Alabama for her lone LPGA Tour title.
''I'm glad to be back in Alabama,'' Thompson said.
Ji, the 2009 U.S. Women's Open champion, had five birdies in a six-hole stretch on the front nine in her bogey-free round.
''I just focus on my game and I just keep positive with my game,'' Ji said.
Korda also had a bogey-free round.
''I was hitting the ball pretty good,'' said Korda, the 2012 Women's Australian Open winner. ''I made two birdies my first two holes and I was like, 'OK, this might be a good day.'''
The second-ranked Lewis opened with a 70.
''I didn't really have anything go right today,'' said Lewis, the winner this year in Singapore and Phoenix. ''I had one ball hit the sprinkler and go in the trees and one hit the cart path and go in the trees.''
Michelle Wie shot a 74.