A stroke ahead after each of the first two rounds, Kim had a 14-under 202 total at Club de Golf Mexico. She won the last of her two LPGA Tour titles in 2005.
''It's a combination of things,'' Kim said. ''I've been really working hard on the game. My caddy, T.J. Jones, and I have been kind of using the mindset every day is Sunday because the previous six weeks in Asia, I've been putting up really good scores on Sunday.
''So, I kind of forget about the Thursday through Saturday bit and, more than anything, I've kind of been gaining inspiration from just being here in Mexico. It's been a long time since I've been back at this tournament. I've just been trying to channel my inner Lorena.''
Kim matched China's Shanshan Feng for the best round of the day in calm, sunny conditions on the tree-lined course. The 30-year-old American birdied two of the first three holes, bogeyed No. 5 and pulled away with birdies on Nos. 10, 13 and 17.
''I'm just here to enjoy myself and really just go out and try and play some good golf,'' Kim said. ''I've come to realize that it's just a game. And that golf is what I do, not who I am. I just turned 30 in March - I know it was like eight months ago, but I still say I just turned 30 - but life has just begun. So for me, this is like my second act.''
Feng was second. She won the LPGA Malaysia last month.
''My putting was good.'' Feng said. ''The greens were a little hard to read, so I missed a couple of birdie chances.''
Munoz struggled on the greens.
''I think something was going on with my stroke because everything was messing up,'' Munoz said. ''Couple I probably misread a little bit, but a couple of them, I missed them. It's been frustrating because I gave myself so many chances, especially on the back nine.''
Top-ranked Inbee Park was sixth at 7 under after a 69. She's coming off a victory two weeks ago in the LPGA Taiwan.
Paula Creamer, second entering the round, had a double bogey and two bogeys in a 76 to drop into a tie for eighth in the 36-player field at 5 under.
Michelle Wie was 3 under after a 71. She won the 2009 event at Guadalajara Country Club for her first LPGA victory.
Second-ranked Stacy Lewis had a 70 to reach even par.