Webb, who won last week's Australian Open at Royal Sydney, will be looking for her sixth win in the Masters on Sunday.
'It was one of the best putting rounds I've had for a long time,' said Webb. 'I think I could have putted with my eyes shut and it still would have gone in. I made just about everything I looked at.'
For most of the day, the others golfers in the field, including second-round leader Ahn Sun-ju of South Korea, could only watch as Webb moved up the leaderboard. When her name finally got to the top of a manual greenside scoreboard off to the side of the 18th green, a large cheer went up in the stands.
Webb and Ellis have 15-under totals of 201. Webb started the day seven shots behind second-round leader Ahn, who had a 72 Saturday to fall into a tie for fourth.
Fellow South Korean Shin Ji-yai was alone in third after a 66, one stroke back of Webb and Ellis.
Ellis, an LPGA veteran, had a 68 Saturday and took a share of the lead when she birdied the par-5 15th, her third birdie in a row. Her 12-footer for birdie on 17 that would have given her the outright lead fell inches short.
Coming back from nearly two years on the sideline because of a serious rib injury and having to cope with the illness of her father, who had a leg amputated last year due to cancer, Ellis hasn't been in many final groups lately on a Sunday.
'I'm not going to say I won't be nervous, because I will,' Ellis said. 'I'll take it as it is and play my game.'
Cristie Kerr of the United States was in the fourth-place tie with Ahn and Lotta Wahlin of Sweden (69) after a 66, three back of Webb. Kerr has finished in the top 10 of 11 straight tournaments.
'I've been consistent, and hopefully I can putt better tomorrow and give myself a chance of winning,' said Kerr, who had 19 top-10 finishes on the LPGA tour last year and won three tournaments.
Webb's birdies came mostly in two spurts -- four of five holes on the front nine, and three of the final four on the back.
On the 17th, a lob pitch from the rough clipped a tree but managed to land on the green. Webb then made a 15-footer for her 10th birdie of the day.
On 18, her attempt to equal her career-best round of 61 fell short when her approach went into the bunker. Her shot out went inches past the flag, and she made the 4-footer coming back to salvage par.
Webb held the previous Royal Pines course record with Japan's Ai Miyazato and American Tiffany Joh. Webb shot 63 in 1999, and Miyazato and Joh matched that in 2005.
There are 11 players within six shots of the lead, including American Brittany Lincicome, who had a 66 Saturday to move to 9-under, tied for eighth.
'I finally got my putter going,' said Lincicome, whose seven birdies all came from within 10 feet. 'I went to the practice green last night and discovered I was standing too far from the ball. The small adjustment made all the difference.'
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