Se Ri Pak, the 2001 champion, fired a 5-under 67 Saturday and is tied with Cristie Kerr, who shot a 3-under 69, in third place. The duo stands at 14-under-par 202.
Annika Sorenstam also carded a 5-under 67 in the third round and is alone in fifth at minus 13. She is now in the hunt for her fourth win in as many starts and her 10th on the LPGA Tour in 2002.
Koch owned a four-shot lead heading into Saturday's third round and collected a birdie and a bogey in her first two holes. She knocked her approach to five feet at the fifth to set up birdie and made it two in a row at six when she wedged her third shot at the par-5 hole to a foot.
At the par-3 eighth, Koch landed a 5-iron on the green, but left her birdie try six feet from the hole. She missed the par-saving putt coming back and made the turn at 1-under 35.
On the back nine, Koch began to struggle with her irons. She missed six greens in regulation in a row, but at every hole, Koch was able to get up and down for par. Her short game and outstanding putting, which seems to have carried over from last month's Solheim Cup, kept her on the top of the leaderboard when others were moving closer.
'I hit some really bad shots around 12, 13 and 14,' said Koch, who won last year's LPGA Corning Classic. 'I started hitting it really bad and I said 'Well, forget it. I'm just going to swing through it and it's just going to go wherever.''
Koch regained her composure in the fairways and gave herself birdie chances at 15, 16 and 17. Her birdie putts narrowly missed the hole, but she left nothing to chance on 18. Koch played her approach to a foot for a tap-in birdie and a two-shot lead.
'It makes a big difference to hit it that close where you can just go up and knock it in,' said Koch, who announced she is expecting her second child. 'It didn't feel like things were going my way today. I'm glad I got up and down a lot of times and stayed patient with it and didn't get too upset with myself.'
The last time Koch took the lead into the final round was at the Women's British Open in July. She posted a final-round 74 and finished in a tie for 10th behind winner Karrie Webb.
'I really didn't feel my game was very good that week,' said Koch, referring to her 54-hole lead at Turnberry Golf Club. 'I feel like my game is much better now than it was when I was leading at the British Open.'
Klein tallied three birdies through her round, but soared up the leaderboard at the par-5 16th. She holed her third shot from 98 yards with a 9-iron to polish off her round of 5-under 67.
Klein has played all three rounds of this event without a bogey.
'I don't have that many bogeys, it doesn't really surprise me,' said Klein. 'When you're hitting the ball well and are having birdie putts all day, you're not thinking about making bogeys. You're thinking about flying it down the flagstick.'
Full-field scores from the LPGA Tournament of Champions