World number-two men's golfer Phil Mickelson of the United States has pulled out of this week's Byron Nelson Championship in Irving, Texas, after his wife Amy was diagnosed with breast cancer.

Mickelson's management company says Amy Mickelson will undergo more tests, but her treatment will include major surgery - possibly within the next two weeks.

Phil Mickelson was set to lead the field at the Byron Nelson -- which tees off Thursday, but he has suspended his PGA Tour schedule indefinitely.

Phil and Amy Mickelson have been married since 1996 and have three children aged 9, 7, and 6.  His withdrawal from the Byron Nelson leaves Vijay Singh of Fiji as the top-ranked player in the tournament.  The ninth-ranked Singh is also a three-time major tournament winner and won the Texas event in 2003.

Mike Weir of Canada, the 2003 Masters champ, 2004 British Open winner Todd Hamilton of the United States and former PGA winner Shaun Michael of the United States are also entered in the Texas event.

Top international players in the field include South African Rory Sabbatini,  Sweden's Jesper Parnevik, Ian Poulter of England and Alex Cejka of Germany.

The tournament is being played on the TPC Four Seasons at Las Colinas, Texas, a par-70 layout that measures 6,552 meters.  The winner earns $1.17 million.