On the day Luke Donald made his debut as golf's new No. 1, Chris Riley and players much farther down the ranking wound up atop the leaderboard Thursday in the Memorial.

Riley, who is No. 280 and was surprised to learn he was even eligible for this event, relied on his putter for eight birdies and a couple of big par saves for a 6-under 66 to take the lead among early starters.

He was one shot clear of Josh Teater (No. 264) and Chris DiMarco, who played on two Presidents Cup and two Ryder Cup teams before injuries and pedestrian play sent him tumbling to No. 375 in the world.

Donald acquitted himself just fine.

Despite a scratchy start, including a double bogey on the 18th when it took him four shots from 35 feet away in a bunker, Donald ran off four straight birdies late in his round for a 2-under 70. Otherwise, it was another solid round, this time with a different ranking.

"It feels good," Donald said. "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."

He played with Masters champion Charl Schwartzel and four-time major champion Phil Mickelson, who each shot 72.

Rickie Fowler, the runner-up at the Memorial last year, played a four-hole stretch on the back nine at 5 under, ending with an eagle on the 15th. He bogeyed the next two holes, then birdied the 18th for a 68, putting him in a group with Steve Stricker and Rocco Mediate.

Even in perfect weather — especially for this tournament — Riley atop the leaderboard was a surprise.

"It was very unexpected," Riley said.

That much was obvious based on his record. Riley had not been to Muirfield Village since 2002, and in three previous appearance, he had never so much as broken par or stuck around for the weekend. Then again, the Memorial is renowned for its wet weather, which makes the course play even longer and look even less attractive to Riley.

Despite massive amounts of rain in April, the forecast is for sunshine all week, and the course is running past and pure.

Teater raced out to a great start at 5 under through seven holes, and despite a few hiccups, kept his round largely in tact. DiMarco also made it through consecutive bogeys with a 31 on the front nine to end his round.

DiMarco can thank an old friend for being at Memorial — tournament host Jack Nicklaus.

His biggest moment in golf came in the 2005 Presidents Cup, when DiMarco holed a 15-foot birdie putt on the final hole to win his match and give the Americans a victory. He rushed into the arms of Nicklaus, the U.S. captain.

As for the exemption.

"Presidents Cup helped," DiMarco said with a grin. "I kept reminding him, 'Remember that putt I made in the Presidents Cup? I could use a spot here.'"

Stricker has never had a top 10 in his 11 trips to Muirfield, an amazing streak that surprised him. With his mind on the U.S. Open two weeks away, he found good form with four straight birdies around the turn.

"I knew I've never really contended here coming," Stricker said. "But I thought I snuck in the top 10 at least once. Looking forward to changing that this week."

Mickelson has never seriously contended on the back nine Sunday, and he'll have to catch up on Friday to get back in the game. His biggest problem in the opening round was failing to birdie any of the par 5s, although one par 5 provided plenty of excitement.

On the 15th, he hooked his tee shot into the trees and, after getting a fortuitous drop from a drain, took the high-risk shot through the pine trees to reach the lower shelf of the fairway, which would leave him only a wedge into the green. Trouble is, the ball took one hop too many over the edge, in thick rough with the ball well below his feet. Instead of gambling with a tough shot to reach the green, Mickelson went low-risk and took a penalty drop. He hit wedge to 5 feet and saved par.