Webb shot an even-par 72 Sunday for a four-round total of 265, tying the Oldsmobile Classic record of 23 under by Lisa Walters in 1998 at Walnut Hills Country Club.
In the process, the Australian star held on for a two-stroke victory over fast-closing Meg Mallon who shot a 66.
"I didn't think anybody had a chance with Karrie today," said Mallon, who has nine comeback victories during her career. "Maybe a bit of complacency held her down."
Webb went into the final round with a commanding eight-stroke lead over Mallon and Cristie Kerr. At the time, it seemed like her biggest task would be breaking her own LPGA record of 26 under, set last year in the Australian Ladies Masters.
But she never could get her game in gear.
In Saturday's third round Webb birdied three of the first four holes. On Sunday, she was struggling right from the start.
"I didn't have a lot of adrenaline today," said Webb, who has six wins this year, including two majors, and leads the LPGA money list. "I was just a little flat. My whole game was flat."
Trouble came on the third hole where Webb drove into the left rough, to a lie that left the ball below her feet. Her second shot rolled over the green, into some rough on the back side. A chip and two putts later, she had her first bogey.
In the early rounds, whenever Webb made a bogey, she responded with a birdie on the very next hole. Not this time.
Webb did birdie No. 6 with a chip within 4 feet, only to give it back with a three-putt bogey at 9 to make the turn at 1-over 37.
Webb's frustration continued on the back side. She was splitting the fairways with powerful drives of 280 yards, but the rest of her game was just so-so.
At the 13th, an uphill par-3, Webb went with a 4-iron off the tee. But she hit it so fat the ball just barely cleared the pond full of ducks fronting the green. Her chip from a mushy lie landed 20 feet from the pin, and Webb needed two putts to get down.
Now she was back to 22 under, and her lead was reduced to just three strokes over Mallon who was playing in the twosome just ahead of Webb and Kerr. And her frustration was beginning to show.
"Swing at it," Webb yelled to herself after leaving her second shot at No. 15 about 35 feet below the pin.
A roar from the crowd up ahead told Webb that Mallon had birdied No. 16 to go 20 under, cutting her lead to two strokes. This time, Webb was up to thchallenge.
A 5-iron on the 175-yard 16th left Webb 15 feet from the pin. As she marked her ball, Webb could see clearly on a greenside leaderboard how close Mallon had drawn. She responded by draining the putt for her last and most important birdie of the day.
"That was the biggest key there," Webb said. "If I didn't make birdie there, it was going to be really tight. That gave me a three-shot lead with two to play which is no guarantee. But it at least made me feel a little more comfortable."
That putt looked even bigger after Mallon hit a wedge inside 3 feet for another birdie at No. 17.
Webb won $112,500, giving her $1.68 million so far this season.
In the process of earning this victory, Webb had a round of 11 under, setting a tournament record and matching the LPGA record by Annika Sorenstam in 1999. Webb's 23 under after 54 holes also was a record, breaking her own mark by one stroke.
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