GULLANE, Scotland (AP) - Mark O'Meara turned back the clock to 1998.
Rory McIlroy can't figure out what he was doing 11 months ago.
While Zach Johnson seized the British Open lead with a 6-under 66 on Thursday, the biggest surprise was O'Meara. He ripped through the front nine at sunny Muirfield as though he was in his prime - not a 56-year-old who has combined to shoot a combined 76 over par over the past decade at golf's oldest major.
The Open champion from 1998 at Birkdale made the turn with a 5-under 31 before stumbling with his first bogey at the 10th, leaving him one stroke behind Johnson.
Not that it's unusual for an old-timer to play well in the Open. Four years ago, Tom Watson nearly won at age 59. Greg Norman led after 54 holes well into his 50s.
The biggest disappointment of the opening round was McIlroy, who only last August won his second major title with a runaway victory at the PGA Championship. He showed no signs of snapping out of his baffling slump this season, struggling mightily to a 79 that marked the second-worst round of his Open career.
The only time McIlroy shot worse was an 80 at St. Andrews in 2010, but that was more a product of a brutal wind than poor shots.
This time, he could blame only himself.
Under brilliant blue skies, the temperature climbed into the 70s and the wind off the Firth of Forth wasn't too much of a hindrance. Sure, the greens were slick as ice, having baked in the unseasonably dry Scottish weather over the past few weeks, but McIlroy has plenty of problems just getting to the green.
Time and again, he found himself whacking at the ball out of the rough or trying to escape the treacherous bunkers. His most telling sequence came at the 15th, where he drove it into the tall grass, chopped it out just short of the green, then sent a putt screaming past the flag - right into a bunker on the other side. He let out a sigh that said everything - a once-dominant player who, as Paul Azinger said earlier in the week, looked "adrift."
"I wish I could stand here and tell you guys what's wrong and how to make it right," McIlroy said. "I don't know what you can do. You just have to try and play your way out. Sometimes I feel like I'm walking out there and I'm unconscious."
An unheralded Spaniard, Rafael Cabrera-Bello, opened with a 67. Three other players - Miguel Angel Jimenez, Brandt Snedeker and Dustin Johnson - were right in the mix after posting 68s. Another shot back were major champions Phil Mickelson, Angel Cabrera and Todd Hamilton.
Mickelson and Cabrera expect to contend in events of this magnitude. Mickelson challenged for the win at the U.S. Open last month before losing to Justin Rose, while Cabrera lost in a Masters playoff to Adam Scott back in April.
Not so much for Hamilton, who won the Open in 2004. The 69 was his lowest round in the tournament since that improbable victory nine years ago.
"This game is a lot about confidence," said Hamilton, who now plays on a minor-league tour in the U.S. "I didn't really know what to expect. I hit a couple of drives early with the driver and made a few putts and that kind of settled me down, and I didn't try to do a lot of stuff that I didn't feel comfortable doing."
It was a day for going low.
"If the wind stays like it is, it's really not too difficult," Hamilton said. "If you can take advantage of the par-5s and throw some other birdies in, and stay away from the high numbers, I think somebody is going to shoot a good score."
Johnson sure took advantage, quickly shaking off his playoff defeat in the John Deere Classic last weekend. He didn't arrive at Muirfield until Monday morning after making bogey on the 72nd hole and losing out to Jordan Spieth, who became the youngest winner on the PGA Tour since 1931.
The 2007 Masters champion made an eagle at the par-5 fifth, then birdied the next two holes to claim the top spot on the board.
Johnson was still there when he walked off the green at No. 18 - another brilliant opening round at the Open after a 65 last year at Lytham.
"I don't know what the secret is," Johnson said. "I hit some nice shots and obviously I putted really, really well."
Perennial favorite Tiger Woods was among those playing in the afternoon, when the greens figured to firm up even more in sunshine so bright it prompted some fans to break out umbrellas to ward off rays rather than rain.
Looking to snap the longest stretch of his pro career without a major title, the world's top-ranked player yanked his opening tee shot off a lone tree far left of the fairway. Woods was forced to take an unplayable lie and settle for a bogey.