SPRINGFIELD, NEW JERSEY (AFP) - Unheralded American golfer Jimmy Walker captured his first major title, outdueling top-ranked defending champion Jason Day down the back nine on Sunday at rain-softened Baltusrol to win the PGA Championship.

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.

"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. "I know how it feels," he said. "It's a very special moment to be able to celebrate on the 18th green."

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.

The 37-year-old from Oklahoma took the Wanamaker Trophy and the top prize of US$1.8 million (S$2.4 million) from a US$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.

American Daniel Summerhays was third on 270, three strokes behind runner-up Day, with Japan's Hideki Matsuyama, South African Brendan Grace and American Brooks Koepka sharing third on 271.

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 organisers 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. It was the right thing to do to showcase the best players in the world."

Groups kept their third-round pairings for the last round in another time-saving bid.

Walker led by one stroke after the morning third round and opened the afternoon final round with nine pars to make the turn at 11 under, one ahead of Day and British Open winner Henrik Stenson of Sweden.

He then sank a stunning 45-foot chip from a greenside bunker to birdie the par-four 10th and reach 12 under. Day, who battled back after bogeys on two of the first three holes, answered with a 22-foot birdie putt at the 11th.

Then Walker responded with a birdie of his own from 30 feet at the 11th, moving two clear of Day with Stenson three adrift.

Day and Walker traded clutch pars from there as the final holes elapsed, Day sinking a clutch 11-foot par putt at 15 to stay two back.

Walker backed off an eight-foot birdie putt twice at the par-five 17th, then rolled the right-breaking putt into the bottom of the cup to reach 14 under. But Day sank a 14-foot eagle putt at the 18th to reach the clubhouse at 13 under with Walker on the 18th fairway.

"I was thinking up 17 if I could make that it would put it out of reach," Walker said. "Sometimes things don't come easy. Nothing in golf comes easy. Jason is a true champion - eagle at 18, I would expect nothing less."

Walker sent his second shot into the right rough but lofted his third onto the green 23 feet from the cup. His first putt ran three feet past the hole but he sank the nerve-jangler to become a major champion.

Day began the week fighting fatigue and illness, having never played the 7,428-yard layout until a practice round on the eve of the event.

"I tried to give it a good run, played pretty decent but couldn't get it done," Day said. "I can't be disappointed. Jimmy played great all week."

Stenson stumbled with a double bogey at 15, ending his hopes of becoming the fourth over-40 player to win multiple majors in the same year. He shot 71 and shared seventh with Germany's Martin Kaymer and American Robert Streb.

"It's tough to play 36 in one day," Stenson said. "I didn't have my best game today and I didn't make a putt all day. I thought I hung in there pretty well considering that."