Dublin - Two-time major winner Rory McIlroy fired a nine-under par 63 on Thursday to seize the lead after the opening round of the PGA Tour's $6.2 million Memorial Tournament.
The 25-year-old McIlroy, of Northern Ireland, is fresh off a victory at the European PGA Championship last week, but is seeking his first win on the US PGA Tour since the 2012 BMW Championship.
"I obviously played really well from tee to green. I holed a lot of putts as well, two really good up and downs at the last two holes," said McIlroy.
Masters winner Bubba Watson, Paul Casey and Chris Kirk each posted six-under 66s to share second place, while Keegan Bradley, Michael Thompson and JB Holmes were tied for fifth at five-under 67 at the Muirfield Village Golf Club.
Hunter Mahan, Ryan Moore and Justin Leonard shared eighth at four-under 68.
World number one Adam Scott of Australia, who also won last week at the Colonial in Texas, Jordan Spieth and Aaron Baddeley were among a group of 10 players to share 11th place on 69.
Scott and McIlroy are playing in the same group for the first two rounds of the tournament hosted by Jack Nicklaus. They are joined by Australian Jason Day, who fired an even-par 72.
McIlroy's round included two eagles on the back nine. The world number six also had seven birdies and one double bogey, on the par-four 14th hole.
"I'm really happy and obviously a really good opening round," said McIlroy. "It could have been even better, apart from a double bogey on 14."
McIlroy's win last week at Wentworth was his sixth European Tour title but first on European soil.
It capped a roller-coaster week in which he announced his split from tennis star Caroline Wozniacki on the eve of the tournament and he came from seven shots off the pace to triumph.
McIlroy opened his superb round on Thursday with a par followed by a 10-foot birdie putt at the second. After converting a nine-foot birdie at the fourth hole, he drained three birdies in a four-hole stretch from the seventh to get to five-under.
His 27-foot eagle putt at the 11th and a 15-foot birdie putt at the 13th got him to eight-under.
Scott overtook Tiger Woods on May 19 to reach number one in the world rankings for the first time, winning the Colonial in his first outing as the number one. He finished with six birdies, one bogey and one double bogey.
Watson caught fire with four birdies over his final five holes to end at six-under.
"Today there was really no wind for the whole round. A little bit of rain yesterday softened it up, so conditions were favorable for good scores," said Watson.
American Phil Mickelson was near the top of the leaderboard until he dropped five shots on the last three holes for a 72Leading scores after Thursday's first round of the $6.2 million USPGA Tour's Memorial Tournament (USA unless noted, par-72):
63 - Rory McIlroy (NIR)
66 - Paul Casey (ENG), Bubba Watson, Chris Kirk
67 - J.B. Holmes, Michael Thompson, Keegan Bradley
68 - Justin Leonard, Hunter Mahan, Ryan Moore
- Charles Howell, Aaron Baddeley (AUS), Adam Scott (AUS), Mark Wilson,
Charley Hoffman, Ben Curtis, Nick Watney, Martin Flores, Kevin Kisner,
70 - Ernie Els (RSA), Lucas Glover, Thomas Aiken
(RSA), Nicholas Thompson, Scott Brown, Kim Hyung-Sung (KOR), Hideki