Michael Campbell of New Zealand carded a 4-under 67 to finish alone in third place.
Tiger Woods looked to be on track in the opening round of a major for quite some time. The top player in the world, whose last major title came at the U.S. Open at Bethpage in 2002, ran home a long birdie putt at the second and rolled in a 4-footer for a birdie at the sixth to jump to minus-2.
The 28-year-old three-putted for a bogey at the seventh but countered with a birdie at the par-3 eighth. Woods found trouble on the back side, however, and missed the green at the 12th on his way to a bogey.
Woods dropped another shot at the 13th after he couldn't get up and down but got back into red figures with a birdie at the par-5 16th. He ended up with a 1-under 70, which ties him for 26th.
'I think it was positive the way I played today. That's the key,' said the champion in 2000 at St. Andrews. 'Granted, I shot a good number, but I think that's indicative of the shots I hit, and I hit a lot of good shots and controlled my ball really well today.'
Casey, who tied for sixth at the Masters, did more than a good job in improving upon his opening round score from last year's Open at Royal St. George's. An 85 on the first day last year meant Casey would be missing out on the weekend for the second year in a row. This time around, the 26-year-old is halfway to making the cut at the Open for the first time in his young career.
'Major championships are a big deal, and that's what everybody wants to win,' said Casey. 'You have to treat them the same as every other golf event and if you don't then 85s happen. You've got to want it a lot, but not push it over the edge and find a happy medium.'
Casey got things going early at the par-4 first and hit a wedge in to 10 feet for a quick birdie. He then knocked his third shot to 6 feet for a birdie at the par-5 fourth and hit his tee shot inside 6 feet for a birdie at the tricky par-3 eighth.
The Arizona State product hit a 7-iron over the back of the green en route to a bogey at the ninth, but responded well with birdies on each of his next two holes to reach 4 under.
Casey parred his next three holes before picking up a birdie at the par-5 16th. At the par-4 closing hole, Casey left an 8-iron within 16 feet of the hole and converted the birdie putt to enter the clubhouse in the lead under calm conditions on the seaside links at Troon.
'I'm not getting in my own way as I have done, especially last year,' said Casey, a three-time winner on the European Tour. 'This year it's been very good. I've been very relaxed and allowed myself to play good golf, and if I'm in that sort of frame of mind, as I was at the Masters, then anything is possible.'
Levet came into this week on a roll after winning the Scottish Open just to earn a spot in the field.
'When you win a tournament you get really lucky,' said Levet, who was not planning on competing this week. 'I just try to put everything on my side to be ready for this week and play good golf.'
On Thursday, Levet seemed to pick up right where he left off at Loch Lomond. He birdied two in a row starting at the par-5 fourth and ran home a long birdie putt at the par-4 seventh.
Levet made it two in a row with a birdie at the eighth, but gave that shot back with a bogey at the par-4 10th. Levet came right back, however, and placed his second shot inside 7 feet for a birdie at the 11th.
He coasted at even par over his next five holes before collecting a birdie at the par-3 17th. Levet then parred the last to join Casey atop the leaderboard.
Levet was on the verge of major glory two years ago at Muirfield when he lost out in a playoff battle to Ernie Els.
'Sometimes you feel like I was not too far from winning. But sometimes you learn from that, as well,' said Levet. 'It's the first time I was about to win something and there is nothing to be ashamed of. He won because he was the best.'
Campbell, who tied for third at the 1995 Open at St. Andrews, tallied a birdie at the fourth and added an eagle at the par-5 sixth. With the front nine yielding plenty of scoring, Campbell then birdied the ninth to make the turn at minus-4.
The 35-year-old bogeyed the 10th but moved back within a shot of the lead with a birdie at the par-4 15th.
'I was very, very comfortable out there today,' said Campbell, who was in contention last week at Loch Lomond. 'It was a good start to the tournament and I'm pretty happy about it.'
Vijay Singh posted a 3-under-par 68 to join amateur Stuart Wilson, K.J. Choi, Gary Evans, Carl Pettersson, Marten Olander, Kenneth Ferrie, Alastair Forsyth and Mathew Goggin in a tie for fifth.
Els had the shot of the tournament thus far at the difficult par-3 eighth. The South African holed a wedge for an ace to reach 3 under and added a birdie at the 11th to join a logjam at 4 under par.
While his challengers faltered over the difficult closing holes at Troon, Els got himself out of some hairy situations. He hit an errant drive at the 13th but was able to muscle his way out of the rough on his way to a par. Els then parred his next three holes before disaster struck at the 17th.
Els found a pot bunker off the tee at the par-3 and left his second shot in the sand. He knocked his third just off the front of the green and what had been a fine morning turned sour after Els two-putted for a double bogey.
'I didn't feel good after that,' said Els. 'I had a pretty nice round going there. But from such a highlight on eighth to such a low light on 17, it's amazing.'
Els played his second shot to 20 feet at the last and two-putted for par and a round of 69.
Colin Montgomerie was eager to get out in front of his hometown fans on his home course. Montgomerie, a member at Troon who has played the course since his youth, rocketed out of the gate with three birdies over his first nine holes.
The Scot stumbled on the back side with a double bogey at the 10th and a bogey at the 11th, but Montgomerie responded with a birdie at the par-4 12th to get back to 1 under.
Montgomerie then sank a 20-footer for a birdie at the par-4 15th and parred his way in for a 69 of his own.
'Whether that score is 75 or 65, I was going to enjoy myself, and I did today,' said Montgomerie, who tied for 24th at Troon in 1997. 'It helps if you break 70, it always helps.'
Els and Montgomerie were joined by U.S. Open champion Retief Goosen, Darren Clarke, Rich Beem, Joakim Haeggman, Kenny Perry, Scott Verplank, Paul McGinley, Skip Kendall, Trevor Immelman, Barry Lane and Steve Lowery at 2 under par.
Justin Leonard, the 1997 winner at Troon, was one shot further back at 1-under-par 70 in the group that featured Woods, 1985 Open champion Sandy Lyle, 1995 winner John Daly, Jay Haas and Robert Allenby.
Masters champion Phil Mickelson was unable to take advantage of the benign conditions on Thursday. He parred his first nine holes before picking up a bogey at the 10th. Mickelson dropped another shot with a bogey at the 15th but recovered with his only birdie of the day at the 16th.
The left-hander was not out of the woods, however, and bogeyed the 17th for a round of 73.
Defending champion Ben Curtis hit his approach to 6 feet for a birdie at the opening hole. It was all downhill from that point on for the Ohio native, who double bogeyed the fifth to fall back to 1 over.
Curtis birdied the very next hole, but collected four bogeys the rest of the way to finish in a group at 4-over-par 75 that included Sergio Garcia.
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