(CNN) -- Padraig Harrington returned to top form ahead of his defense of the PGA Championship with a six-under-par 64 to lead the Bridgestone International.
Harrington grabbed six birdies in a flawless display at Firestone Country Club.
The Irishman has endured a miserable year after reshaping his swing, but his first round showing at Firestone Country Club in Ohio showed his hard work may be paying off.
"Compared to the first six months of the year it's obviously my low score of the year, my best score of the year," he told reporters.
"The first six months had a purpose, I was working on things and I suppose it's time now to go and play some golf."
Harrington carded six birdies on the tough par-70 South Course to lead playing partner Scott Verplank of the USA and early pacesetters Prayad Marksaeng of Thailand and South Africa's Tim Clark by two shots.
England's Ian Poulter is a further shot behind at three-under 67 with Americans Steve Stricker and Zach Johnson.
Six-time Firestone winner Tiger Woods is in a six-strong group on 68 as he tunes up ahead of the final major of the season at Hazeltine next week.
"I hit a couple of bad shots starting out, and got it back in the middle part of the round," Woods said.
Stewart Cink, playing his first tournament since winning the British Open at Turnberry earlier this month, opened with a one-under 69 in a group that also includes U.S.Open champion Lucas Glover.
Defending champion Vijay Singh, world number two Phil Mickelson and Masters champion Angel Cabrera were in a large group on level-par 70.
Mickelson is playing his first tournament in six weeks as he cares for his wife Amy, who is battling breast cancer.