Casey relishing strong start
Paul Casey finished last year's US PGA Championship after two days with a round of 85, but he began the final major of 2013 with a superb 67.
The former world number three, down at 169th in the rankings after struggling for form and fitness before winning the Irish Open in June, carded five birdies and two bogeys to finish three under at Oak Hill on Thursday, two behind clubhouse leader Jim Furyk.
World number one Tiger Woods, seeking his 15th major title but a first since the 2008 US Open, was two under after six holes but eventually finished one over after a double-bogey six on the ninth, his final hole.
Defending champion Rory McIlroy was showing some of the swagger he had been looking for by watching videos of last year's eight-shot triumph at Kiawah Island, the world number three reaching the turn in three under largely thanks to three birdies in his first four holes.
Lee Westwood, seeking a first major title at the 63rd attempt after letting slip a three-shot lead in the final round of the Open, was three under after 11, while Masters champion Adam Scott carded five birdies in a row from the fourth to race to the turn in 30.
Casey had started inauspiciously with a bogey on the 10th and followed that with eight straight pars, but then birdied five of his next seven holes and made sure to check out his name high on the leaderboard as he finished.
"It's a while since my name has been up there," said the 36-year-old, who broke his collarbone snowboarding at the end of 2011. "It felt good.
"You don't realise how much you miss something until it's gone and sitting on the sofa watching the big events of the last year and a half I have had lots of extra motivation to get back.
"It added another reason for getting out there and working hard and things like Justin (Rose) winning the US Open is good fuel for the fire.
"I got a bit lucky today with a couple of monster birdie putts from 50ft and 33ft on the fifth and seventh, but we're off and running. I knew I had to get something going after the turn because I had got through the tough part of the course."
The early starters enjoyed perfect scoring conditions with an overnight thunderstorm softening the course and Woods - fresh from winning his eighth WGC Bridgestone Invitational on Sunday by seven shots - was two under par after six.
He looked certain to move three under after a superb approach to the second - his 11th - but the birdie attempt from three feet never even touched the hole.
A poor tee shot on the par-five fourth - the first time he had used a driver - contributed to a first bogey and worse was to follow on the ninth, where he came up short of the green and then pitched into the face of a greenside bunker.
"I'm still right there," Woods insisted. "As of right now I'm only six back and we have got a long way to go. I played really well today. The round realistically could have been under par easily."
Furyk, looking to become the second 43-year-old in succession to win a major after Phil Mickelson's Open triumph, was set to equal the course record of 64 when he reached six under, but bogeyed the last for a five-under 65.
The former US Open champion led by one from Canada's David Hearn, with Casey joined on three under by American pair Matt Kuchar and Robert Garrigus and Australia's Marcus Fraser.
McIlroy duffed a chip on the 10th and failed to get up and down from short of the 11th to card back-to-back bogeys before play was suspended at 4:25pm due to the threat of lightning from an approaching storm.
Westwood remained three under after 12 but would have a short birdie putt on the 13th on any resumption, while Spain's Miguel Angel Jimenez was four under with six to play.
Tournament officials announced their intention to resume play at 5:35pm, a delay of 70 minutes which meant the first round could be completed on schedule this evening.