Rory McIlroy leads the Memorial Tournament by three shots after carding a nine-under-par 63 on the opening day in Dublin, Ohio.
McIlroy followed up his victory in the PGA Championship last week with an impressive round despite sustaining a knee injury on the seventh hole.
England's Paul Casey, Masters champion Bubba Watson and fellow American Chris Kirk are tied at six under.
Luke Donald is one under par, while Justin Rose shot one over.
McIlroy, who announced his split from fiancee Caroline Wozniacki at Wentworth, carded seven birdies, two eagles and a double bogey on the 14th despite struggling with the onset of a knee problem.
He told the Golf Channel: "I felt it on my second shot on seven. My foot might have got stuck in the ground. I'll have a physio look at it. It was painful after I hit shots and I sort of walked it off before the next one.
"This is a new injury. I've never really struggled with my knees before. I tweaked my back last Friday at Wentworth, I'm still struggling with that.
"I guess I put it out of my mind when I was over the golf shots and pretended it wasn't there and tried to hit the best shots I could."
McIlroy, 25, moved up to sixth in the world rankings with his win at Wentworth and continued his fine form by picking up seven shots in his first 11 holes.
He responded to a double bogey on the 14th with a second eagle of the day a hole later to move clear at the top of the leaderboard.
Five-time major champion Phil Mickelson ended with two double bogeys to drop five shots in the last three holes to finish with a level-par 72.