DUBLIN, Ohio - Rory McIlroy only gets to play three regular PGA Tour events this year. He showed Thursday why the Memorial Tournament was one of them.

On a Muirfield Village course that already ranks among his favorites, McIlroy had a birdie putt on his last eight holes and converted half of them on his way to a 6-under 66 to join Chris Riley in the lead after the first round.

McIlroy hit the ball so pure that he shot 32 on the front nine despite missing three birdie putts inside eight feet.

The 22-year-old from Northern Ireland narrowly made the cut at Wentworth in the BMW PGA Championship and wound up in a tie for 24th. Asked the difference between last week and this week, he replied: "It's about 30 degrees warmer."

A tournament known for its sloppy weather has been spectacular, and it showed in the scoring on a well-manicured course.

Chris DiMarco and Josh Teater were at 67, followed by a large group at 68 that included Steve Stricker; Dustin Johnson; Stewart Cink; and Rickie Fowler, the runner-up at the Memorial last year.

Fifty-one players in the 120-man field broke par.

Luke Donald, in his debut as the No. 1 player in the world rankings, recovered from a slow start by making four straight birdies toward the end of his round for a 70. He played with Masters champion Charl Schwartzel and four-time major champion Phil Mickelson, who each had a 72. Mickelson did that without making a single birdie on the par-5s.

This is the last tournament for McIlroy before the U.S. Open, a chance for the No. 6 player in the world to atone for his 80 in the final round that cost him a chance to win the Masters.

And now, the U.S. Open beckons - along with a chance to win on another stronger course in America.

"This is one of my favorite weeks of the year, one of my favorite courses," McIlroy said. "I feel as if it really does set up well for me. I like these sort of golf courses, the likes of here and Akron (Firestone) and Quail Hollow. And I'm swinging well, I'm hitting it good, and I'm holing a few putts."

West Chester's Sean O'Hair shot a 4-over-par 76, which left him tied for 95th.

For Donald, it felt slightly different from other tournaments, just because of the ranking attached to his name.

"I'm excited to be there and looking forward to the challenges. I heard a few 'No. 1' shouts and stuff like that, so you feed off that."

European PGA

NEWPORT, Wales - South African journeyman Keith Horne led the Wales Open after shooting a 7-under 64 in the opening round, with defending champion Graeme McDowell trailing by 3 strokes at Celtic Manor.