Trading dramatic shots over the final holes of a 36-hole golfing marathon, Walker and Australian star Day kept the drama intense to the last shot, a three-foot par putt by Walker at the 18th to claim a one-stroke victory yesterday.
"Sometimes a par is tough," Walker said. There were a lot of emotions out there... It was a battle all day."
Day was at the 18th green to congratulate Walker after the tension-packed putt.
Walker, a former top-10 player now set to rise from 48th in the rankings, fired a three-under par 67 in the final round to finish 72 holes on 14-under 266 and win his sixth US PGA crown, the first since last year's Texas Open some 16 months ago.
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The 37-year-old from Oklahoma took the Wanamaker Trophy and the top prize of $1.8 million (1.6 million euros) from a $10 million purse.
Walker, the PGA's first wire-to-wire winner since Phil Mickelson at Baltusrol in 2005, missed the cut in three of the prior four majors and was never better than seventh in 17 prior major starts.
Day and Walker were among 10 golfers forced to play their entire third and fourth rounds on an overcast and sometimes rainy Sunday after thunderstorms halted play Saturday.
The water-soaked course nudged organizers into using preferred lies, or "lift, clean and place" rules, in the last round, believed to be a first in major golf history. Players could clean mud off their balls in low-mown areas and replace them.
"It's certainly a difficult decision," PGA of America chief championships officer Kerry Haigh said. "We just felt for the fair play of a major championship we needed to play preferred lies in the final round.
Groups kept their third-round pairings for the last round in another time-saving bid.