(Reuters) - Top seed Dustin Johnson continued his "good mojo" to earn a tie for the first-round lead with Kevin Chappell and Hideki Matsuyama at the Tour Championship in Atlanta on Thursday.

The power-hitting U.S. Open champion continued to make a fickle game look easy, compiling five birdies in a seemingly-effortless four-under-par 66 in ideal conditions at East Lake.

"I felt like I had good mojo going all day and swung it very nicely," Johnson told Golf Channel. "I hit a lot of great shots with the irons and drove it nicely. That puts together a good score."

Johnson, fellow American Chappell and Japan's Matsuyama headed Australian world number one Jason Day, South Korean Kim Si-woo and American Kevin Kisner by one stroke in the elite 30-man field.

Day continued to feel occasional twinges of the back pain that forced him to pull out of the previous playoff event, the BMW Championship, after three rounds.

The season-ending Tour Championship is the only event on tour that hands out two trophies -- one to the tournament winner and another to the winner of the season-long FedExCup points race.

The FedExCup champion will receive $10 million and Johnson, a three-time winner this season, is in pole position. He will win the FedExCup if he also wins the tournament, as would the next four seeds -- Patrick Reed, Day, Adam Scott and Paul Casey.

Casey made a 30-foot eagle putt at the par-five 18th for two-under 68, while Scott finished poorly with two late bogeys for a 69 and Reed struggled to a 73.

Johnson hit a couple of wayward drives, but was otherwise a model of consistency.

His sole bogey came at the par-four 13th, his drive there ending up behind a pine cone from where he could advance the ball only 30 yards.

Four holes later, Johnson's drive almost struck Sky Sports on-course reporter Wayne Riley, a former professional who took his on-course role a little too literally by wandering down the fairway instead of staying in the rough.

Johnson's drive landed within a couple of paces of a surprised Riley, who came in for some light-hearted ribbing by his fellow commentators.

Joint leader Matsuyama, meanwhile, made a nice start on the more difficult front nine and picked up two more shots on the inward half.

"I was a little bit lucky on that front nine but got into a good rhythm," said the world number 18. "I putted well today and that's what made the difference."

(Reporting by Andrew Both in Cary, North Carolina; Editing by Mark Lamport-Stokes)