-6 P Harrington (Ire) -4 P Marksaeng (Tha), T Clark (SA), S Verplank (US) -3 S Stricker (US), I Poulter (Eng), Z Johnson (US) Selected others: -2 T Woods (US) -1 L Westwood (Eng), N Dougherty (Eng), O Wilson (Eng) Level R Fisher (Eng), L Donald (Eng), P Mickelson (US)
Harrington defends his US PGA Championship title next week
Padraig Harrington put his indifferent season behind him with a sparkling round of 64 in the first round of the World Golf Championship event in Ohio.
The Irishman has been struggling with his remodelled swing but delivered a bogey-free round at the Firestone Country Club to take a two-shot lead.
England's Ian Poulter is three shots back at three under, while Tiger Woods lurks at two under.
Paul Casey was forced to pull out after six holes with a rib injury.
The world number three, who suffered the injury before the Open, must now be a serious doubt for the USPGA Championship, which begins on 13 August.
"I think I strained an intercostal muscle," said the 32-year-old.
"I actually did it a couple of weeks ago, a week before the Open, hitting drivers.
The first six months had a purpose... I suppose it's time now to go and play some golf
"I battled through the Open and haven't done much in a couple of weeks. I tried to rest it and then tweaked it in the rough on Monday.
"I was even par for six holes and I wasn't doing too bad but it just felt so bad."
Harrington will defend his US PGA title at Hazeltine in Minnesota next week, and his performance on Thursday will provide a major boost.
The 37-year-old rolled in putts from more than 20 feet on the last three holes to end the day two ahead of Thailand's Prayad Marksaeng, American Scott Verplank and South African Tim Clark.
"Compared to the first six months of the year, it's obviously my low score of the year, my best score of the year," said Harrington.
"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.
"The last couple of weeks I was getting more focused on my overall game when I was at home than maybe the first six months when I was focused on my swing."
Poulter shares fifth place with Americans Steve Stricker and Zach Johnson, with Woods lying a shot back with five others including Spanish pair Sergio Garcia and Miguel Angel Jimenez.
Open champion Stewart Cink is in a large group at one under that also features English trio Nick Dougherty, Lee Westwood and Oliver Wilson.
Phil Mickelson, in his first tournament for six weeks, is level par.
The American world number two has been looking after his ill wife and mother, but returned for the prestigious tournament in Ohio.
Also at level are England's Luke Donald and Ross Fisher, who admitted he had come into the tournament short of practice following the birth of his first child.
"I hardly played at all to be honest," Fisher said.
"I had two days of practice at home so not as tournament prepared as I would have liked to have been but obviously circumstances have changed at home so that is the way it is at the moment."