Pressel played her final 25 holes over Mission Hills without a bogey, finishing the round with a 10-foot birdie putt that looked as though it would be only good enough for second place.
Pettersen, a fiery 25-year-old from Norway, had a four-shot lead with four holes to play when she started hitting tee shots into the ankle deep rough and missing putts on the crusty greens. She went bogey-double bogey-bogey to fall one shot behind, and needing a birdie on the par-5 18th, she hit wedge some 25 feet beyond the hole and missed the putt.
Pressel, who finished at 3-under 285, was on the practice range when she entered the history books.
Previously, the youngest major champion was Sandra Post, who was 20 years, 19 days when she won the 1968 LPGA Championship. On the men's tour, Young Tom Morris was 17 when he won the 1868 British Open.
The kid broke down in tears again, this time over the shock and euphoria of winning a major 10 months after graduating high school.
'Oh my God! Oh my God!' was all she could manage.
Then came the sweetest pool party she ever attended. Keeping with tradition at the Kraft Nabisco, Pressel jumped into the pond surrounding the 18th green with her caddie and grandmother, Evelyn Krickstein.
Herb Krickstein, her grandfather and the father of former tennis player Aaron Krickstein, watched with a broad smile. Pressel came out of the water and slipped into a white robe that read, '2007 Kraft Nabisco Champion' on the back.
It was hard to believe -- not only at that age, but how it all unfolded.
Se Ri Pak, needed this major to complete the career Grand Slam, had a three-shot lead on the front nine until Pettersen took charge with a four-shot swing over three holes. Pak bogeyed five of the last six holes for a 77.
Catriona Matthew of Scotland, playing only her second event since becoming a mother three months ago, had a 30-foot birdie putt to reach 4 under when she three-putted for a bogey that left her in a tie for second with Pettersen and Brittany Lincicome (72).
But no one threw this tournament away more than Pettersen.
'I said yesterday that the one who made the fewest mistakes would win,' she said. 'I did a few too many.'
It started with a tee shot into the right rough on the 15th, keeping her from reaching the green and taking bogey. She went right again on the next hole, clipping a branch on her second shot that left her short of the green. Her wedge spun off the front of the green, and she used putter to ram it 8 feet by. The bogey putt caught the lip, and suddenly she was down to 3 under.
Pressel watched workers change Pettersen's score under par from a '5' to a '3' and couldn't believe it. Pettersen then came up short on the 17th, chipped to 10 feet and missed again, falling one shot behind.
A week ago, Pettersen was runner-up to Lorena Ochoa outside Phoenix.
'This time, I felt like I lost the tournament,' she said. 'Last week, I felt like I won second place. Apparently, it wasn't my week. I just didn't finish it off. All credit to Pressel.'
Stacy Lewis was low amateur after a 71 that put her in tie for fifth.
Pressel became the first American to win this major since Dottie Pepper in 1999. And it atoned for Cherry Hills two years ago, when she was poised to win the U.S. Women's Open until Birdie Kim holed a bunker shot for birdie as Pressel watched in disbelief from the fairway.
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