Ted Purdy, winner last week at the Byron Nelson Championship, had a 66. Phil Mickelson, the highest-ranked player in the field at No. 4 in the world, settled for a 71.
Sheehan, whose best PGA Tour finish has been a tie for second last year in San Antonio, chipped in twice during the opening seven holes and made four putts of 20 feet or more.
Needing only a par at the par-4 ninth to shoot a 28 on the front nine, Sheehan put his tee shot into a bunker left of the fairway. He decided to lay up short of the pond that guards the green, but his layup attempt hopped all the way across the fairway into another bunker.
He was forced to lay up again and wound up with a double bogey.
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