One of golf's brightest prospects, McIlroy started his romp with an eagle at the 352-yard ninth, where he drove the green and rolled in the putt. He then made six birdies on the homeward holes for a 30 that tied the tournament's back-nine record.
McIlroy became only the eighth player to go so low at the British Open, equaling a mark from 17 years ago by the late Payne Stewart at Royal St. George. Overall, just 22 players have shot 63 in one of the four major tournaments, including Greg Norman and Vijay Singh, who each did it twice.
The last to do it: Tiger Woods at the 2007 U.S. PGA Championship.
"I didn't get off to a flying start," McIlroy said. "The eagle on 9 really sort of turned things around for me, and I just got going from there. It was great to get into the rhythm of the round and get into a flow. And yeah, it was a very pleasant round of golf."
McIlroy had a chance to claim the record all to himself.
A brilliant approach at No. 17, the famed "Road Hole," left him with a 5-footer for birdie. But the putt slid wide of the cup, and McIlroy rolled back his head in disappointment after tapping in the par, knowing that he had squandered a chance at a truly historic score.
While no one ever expects to shoot 63 in a major, it was certainly a day for posting a low number.
Louis Oosthuizen of South Africa took a run at McIlroy until he failed to get up-and-down at 17, taking his only bogey on the way to a 65. John Daly matched the best score of his Open career with a 66, a number equaled by Scottish journeyman Andrew Coltart, England's Steven Tiley and Bradley Dredge of Wales.
The birthplace of golf played like a public course for the morning starters, with hardly any wind blowing in off St. Andrews Bay and only a sprinkling of rain. In fact, the sun popped out just as McIlroy was finishing up, and competitors were able to strip off their jackets and play in short sleeves.
"The old lady had no clothes on today," said 60-year-old Tom Watson, who opened with a 73 and was one of the few early starters who failed to break par.
Winds stiffened as the day went on, making it more difficult for afternoon players like Phil Mickelson, who was 2 over through 13 holes. Then, just past 6 o'clock, the expected rains finally arrived, at least on part of the course, forcing players to scramble for rain gear.
Tougher conditions didn't seem to bother England's Lee Westwood, who recently was diagnosed with a ruptured muscle in his right calf and couldn't get in much practice time. He was 5 under with three holes to play and solidly in contention on a scoreboard filled with red numbers.
Woods shot a 67, showing this might be the week for his first victory since being tarnished by a sex scandal that had the British media speculating as much about his personal life as the prospects of him becoming the first player to win three Open titles at St. Andrews.
The world's top-ranked player romped to dominating wins in 2000 and 2005; a third straight would give him 15 majors titles, just three shy of Jack Nicklaus' career record.
"It's getting better every week," Woods said. "I'm hitting shots that I haven't hit in a long time. It's building."
Daly knows a thing or two about problems away from the course, from failed marriages and financial woes to trouble with alcohol and weight.
He underwent surgery to deal with his ballooning waist line, and has lost nearly 100 pounds (45 kilograms). But his game had shown little signs of turning around he's ranked 455th in the world and his best finish of the year was a tie for 24th at the Puerto Rico Open.
No matter, Daly still has quite the following. Fans of "grip it and rip it" had no trouble spotting him at the Old Course, where he wore lavender paisley pants, a sky blue sleeveless sweater, a peach shirt and a turquoise cap. None of it matched. Not that it mattered.
Strolling the course puffing on a cigarette and carrying a diet soda, Daly put up his best score since a 66 at Royal St. George in 1993. It could have been a lot better, too four putts lipped out or caught the edge of the cup, including one that spun 180 degrees around the back of the 12th hole to end a streak of four straight birdies.
Could it be 1995 all over again?
That year, Daly managed to put aside his problems long enough to capture his second major championship, beating Costantino Rocca in a British Open playoff with a mop of blond hair that was much longer than it is now.
"It's a golf course, I don't know, I just love it. I don't know why," Daly said. "It suits my game. Just a special, special place."
But the Old Course relies on the elements to provide its best defense. Without the usual rain and wind, it was like target practice for the world's best.
Of the first 78 players to finish, only 22 posted scores above par. Among those in the red: Defending Open champion Stewart Cink, who got off to what normally would have been a very solid start.
This day, though, a 70 felt like an opportunity squandered.
"It was out there for the taking," said Cink, who beat Watson in a playoff at Turnberry after the five-time champion missed an 8-foot putt on the 72nd hole that would have made him the oldest major champion in golf history. "I definitely left a few out there."