updated 6:33 p.m. ET May 20, 2009

Phil Mickelson was gearing up for his favorite time of year, working his way toward Bethpage Black and another crack at the U.S. Open before a New York gallery that treats him like a rock star.

All that changed Wednesday, along with his priorities, when he disclosed that his wife, Amy, has been diagnosed with breast cancer.

Mickelson is taking an indefinite leave from the PGA Tour. He withdrew from the Byron Nelson Championship, which he won in 1996. He is to defend his title next week at Colonial, but even that is uncertain. A statement from his management company said his 37-year-old wife would have more tests, though treatment would begin with “major surgery” as early as the next two weeks.

“We see Amy as this vibrant, bubbly mother of three who is tremendously devoted to her husband and family,” Jack Nicklaus said. “No one, especially Amy, deserves to have to face the battle that accompanies cancer. But we know that Amy has this amazing inner strength and spirit, and with Phil’s unwavering love and support, they will fight and overcome this.”

Mickelson, a three-time major champion with 36 career PGA Tour victories, was closing in on the No. 1 ranking held by Tiger Woods. He was runner-up to Woods at Bethpage Black in 2002.

“Elin and I are deeply saddened to hear the news about Amy,” Woods said. “Our thoughts and prayers are with her, Phil, the children and the entire Mickelson family.”

Scott Verplank said Mickelson sent him a text message Tuesday night “and I had a hard time sleeping.”

“Every time I’ve been around her, she’s always had a smile on her face. She’s always upbeat,” Verplank said. “She’s a neat girl. Hopefully, it’s early and hopefully, they take care of it.”

Interactive Fact file: Breast cancer Learn the basics about the disease, from risk factors to symptoms and more. msnbc.com

Amy Mickelson is one of the most visible wives on the PGA Tour, a former Phoenix Suns cheerleader who regularly walks the course during rounds and mingles easily with fans who recognize her blonde hair and engaging smile.

They met in 1992, when Mickelson was a senior at Arizona State, a year after he won his first PGA Tour event as an amateur. Amy knew nothing about golf at the time.

“I grew up in a tennis family, and when he told me he was a pro golfer, I thought he worked in the shop at a golf course,” she wrote in Mickelson’s book, “One Magical Sunday,” after he won his first major at the 2004 Masters.

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The first time she accompanied him to a golf tournament, the Bob Hope Classic, she figured they would walk hand-in-hand down the fairway and was angry at him for not spending enough time with her. But once she learned the difference between birdies and bogeys, she has been at his side during the highs and lows of golf tournaments.

They were married in 1996 and have three children: Amanda, 9, Sophia, 7, and Evan, 6. Their first child was born the day after the 1999 U.S. Open at Pinehurst No. 2., where Mickelson carried a pager and promised to leave if his wife went into labor.

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