Northern Ireland's Rory McIlroy birdied three of his last four holes to earn a share of the lead at the Memorial Tournament at Muirfield Village.
McIlroy shares the lead on six under with Chris Riley after firing a round of 66 that included seven birdies.
The pair are a shot clear of United States trio Chris DiMarco, Josh Teater and Scott Stallings
World number one Luke Donald made a solid start, hitting a round of 70 to lie four shots off the lead.
Englishman Donald's first round included six birdies, two bogeys and a double bogey on the 18th.
United States duo Steve Stricker and Rickie Fowler were among seven players who ended on 68. Phil Mickelson failed to birdie any of the par fives and shot 72.