Webb also rallied to win the Women's Australian Open last month and has 41 LPGA Tour victories to match founding player Babe Zaharias for 10th place.
Ko, the third-round leader, parred the final three holes to finish a stroke back along with 2013 winner Stacy Lewis, Azahara Munoz, Amy Yang and Mirim Lee.
The 16-year-old Ko shot 70. She had a three-stroke lead after birdieing Nos. 2-5, slipped back with bogeys on Nos. 6, 9 and 11 and pulled within one with a birdie on the par-5 15th.
"I tried to get myself together," Ko said. "I made some bogeys at the wrong time, which wasn't ideal, but I tried my best until the last."
Munoz and Yang, playing together in the third-to-last group, missed long birdie putts on 18 and each shot 67. Lee shot 69 in the next group, also missing a long birdie try.
That left Ko — and her 25-foot try on the last was just short and right.
"I played really well overall, so I'm just going to take the positive out of it," Ko said.
Lewis birdied the final two holes for a 66.
"It was just really nice to hit the shots and make the putts at the end," Lewis said. "Seeing putts go in always helps going into the next couple of weeks."
DIVOTS: Korda, playing with Ko in the last group, had a quadruple-bogey 8 on the par-4 seventh after hitting her approach into the desert, then played the final 11 holes in 5 under to tie for seventh at 17 under. The Bahamas winner shot 70. ... Hee-Won Han also matched the course record with a 63 to tie for 15th at 14 under. Playing in the ninth group of the day, she holed a sand wedge for eagle on No. 7. Ai Miyazato set the course record last year and Cristie Kerr matched it Saturday. Kerr finished with a 67 to tie for 10th at 16 under. ... Top-ranked Inbee Park closed with a 67 to tie for 10th.