(Reuters) - Four weeks after winning his first PGA Tour title, William McGirt continued to ride a red-hot putter as he surged into a three-stroke lead in Thursday's opening round of the WGC-Bridgestone Invitational in Akron, Ohio.

The 37-year-old American stole the limelight from the game's biggest names competing in the elite World Golf Championships event, firing a flawless six-under-par 64 on a breezy, sun-splashed day at Firestone Country Club.

McGirt, who won the Memorial title earlier this month, sank two birdie putts from 30 feet, holed out from a bunker at the 15th and ended his round by draining a 46-footer for par from just off the 18th green.

Australian world number one Jason Day, bidding for a fourth PGA Tour victory this season, opened with a 67 to end the round level with Argentina's Emiliano Grillo and American Jimmy Walker.

"I felt like I did all right," Day told Golf Channel after mixing four birdies with a lone bogey. "For the most part, I hit a lot of good quality shots, iron shots, into the greens and gave myself some opportunities out there.

"When I was kind of out of position, my short game saved me and I had some pretty nice up-and-downs to really keep the momentum going."

American world number two Jordan Spieth endured a poor day on a challenging course from tee to green, but relied on his putting brilliance to card a two-under 68, highlighted by birdies on his last four holes.

"I was making some nice par saves to grind it out and then finally those par saves turned into birdie putts," said Spieth, who hit only seven of 18 greens in regulation.

"I won the battle in my head today. I really could have shot four or five-over pretty easily with the way that I was striking the ball tee to green."

McGirt racked up six birdies, including four in his first eight holes.

"I hit a lot of good, solid shots and fortunately the putter was working for me," said the journeyman from North Carolina. "The hole looked a lot bigger from 30-40 feet than it did from six or eight feet.

"I missed a couple of short ones on the front nine, but I had eight putts on the back nine with three hole-outs. I don't think I have ever had this much confidence in my putter for this long."

Newly crowned U.S. Open champion Dustin Johnson opened with a 69 while world number five Rickie Fowler carded 68 to finish level with fellow Americans Spieth and Charley Hoffman, and India's Anirban Lahiri.

(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue/Larry Fine)