Ireland's Padraig Harrington carded his lowest score of the year, a bogey-free six-under par 64, to grab a two-stroke lead after Thursday's opening round of the Bridgestone Invitational.

Harrington, who defends his PGA Championship title next week, took advantage of relatively benign afternoon conditions to reel off six birdies in the $8.5-million World Golf Championships event at Firestone Country Club.

South African Tim Clark, American Scott Verplank and Thai Prayad Marksaeng were equal second on 66 while world number one Tiger Woods, a six-time winner, did himself no harm with a 68.

Harrington, last year?s British Open and PGA Championship winner, has been mired in a slump for most of this season while tinkering with his swing, so his good start was obviously a step in the right direction.

However, the Irishman refused to assign too much significance to it.

"It has no relevance in the overall scheme of things," he said. "It?s just a round of golf. It shouldn?t affect how I feel about my game at any stage going forward. It doesn?t change the last six months and it won?t change the next six months."

Nevertheless, Harrington had no complaints about his score.

"I played the first 11 holes particularly well. I missed a short birdie putt on (my second hole) and at that stage I was still wondering which way it was going to go," he said.

"On (No. 4) I missed the green and made a very good up-and-down and I think that was important to my round. I felt I was going forward from there.

"Every player out there would love to tell you that how we play the first couple of holes of any tournament has no bearing on the 72-hole result, because we?re all so professional. But it does have an effect.

"This is the round I can remember back to the PGA (last August) that I think I got the most out of. If I had shot 66, I would have said that was about right, so 64 is a little bonus."

Clark, meanwhile, expressed approval with the course set-up, particularly the rough, which is less penal than in some previous years.

"I used to get quite frustrated around here because I?d hit a lot of good tee shots and ball would just kick into the rough," he said.

"The way the rough is now, you can just miss the fairways and still have a chance to get it up around the green. I did have a couple of three-putts, but I still feel like I putted fairway well."

Woods wasn?t at his sparkling best, but he managed his game well enough to record just one bogey and end the day equal eighth.

"I hit a couple of bad shots starting out, and got it back in the middle part of the round," Woods said.

"I hit some good shots on the back nine. Some of the things I was working on Monday and Tuesday at home I put together for a little bit in stretches."

Twenty-six players broke par on a day when the field averaged 70.8 strokes.

Englishman Paul Casey pulled out after six holes with a pulled muscle below his rib cage, cutting the field to 79 players. He received $35 250 that will not count on the official money list.