The American has been suffering with a neck injury over the past two months and had been advised to take a complete rest from playing, but decided to tee-off at the Valhalla Golf Club in an effort to defend his title.

But the 37-year-old was clearly struggling and after triple-bogeying the 10th hole leaving him at eight-over par, he shook hands with playing partners Keegan Bradley of the United States and South Korean Yang Yong-eun before pulling out.

"It's an arthritic, degenerative issue that I'm probably going to be dealing with for a good bit," revealed Dufner.

"It's just something that's not going to change.

"I'm going to have to deal with it."

While Dufner was struggling, Westwood was setting the pace after shooting his joint lowest round in a major with a 65.

The former world number one began his round at the 10th hole with a double-bogey but steadied the ship before going on to rattle in five birdies in his final six holes to finish the day at six-under par.

"I played well, hit a lot fairways, putted nicely," said the 41-year-old.

"I gave myself a lot of chances.

"All in all, there were no real weaknesses out there.

"I hit a lot of quality iron shots, and it felt like 65 was a fair enough score for the way I played."

Jason Dufner was forced to pull out of his USPGA Championship title defence with a neck injury ©Getty Images



World number 107 Chappell, yet to win on the PGA Tour, was a model of consistency in Louisville making three birdies on the front and back nine, while Palmer briefly held the outright lead on seven-under, but a bogey on his penultimate hole saw him drop back to a share of the lead.

Meanwhile, world number one and tournament favourite Rory McIlroy is nicely placed just one shot behind the leaders as he chases a fourth major title and his third win in a row after following up his Open Championship at Royal Liverpool last month with a win at the WGC-Bridgestone Invitational last weekend.

The Northern Irishman reached the turn in three-under but dropped back to level-par after a double-bogey at the 10th and another dropped shot at the 11th.

But the 25-year-old demonstrated his powers of recovery by reeling off four straight birdies before narrowly missing an eagle putt at the last that would have given him a share of the lead.

McIlroy is joined on five-under by American Jim Furyk, Italy's Edoardo Molinari and Henrik Stenson of Sweden, while Finland's Mikko Ilonen and Jerry Kelly of the United States lie a further shot back.

The other two major winners this year, US Masters champion Bubba Watson of the United States and Germany's US Open champion Martin Kaymer, both signed for one-under par rounds of 70.

World number one Rory McIlroy has continued his fine recent form and lies just one shot off the lead at Valhalla Golf Club ©Getty Images



Tiger Woods continues to struggle with form and injury, however, as he opened up with a 74, leaving him nine shots off the leaders.

The 38-year-old was a major doubt coming into the event after aggravating a back injury last weekend and his three-under par round included four bogeys and just one birdie.

"It wasn't very good," said Woods, chasing a first major win since 2008.

"A lot of bad shots and I never got a putt to the hole.

"For some reason, I thought they (greens) were going to be a little bit quicker and I didn't make the adjustment well enough.

"My swing was dialled in on that (practice) range out there.

"Unfortunately, I didn't carry it to the golf course."

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