Thailand's Prayad Marksaeng and Tim Clark of South Africa set the early pace at the World Golf Championships-Bridgestone Invitational as the first wave of starters completed their opening rounds at Firestone Country Club in Ohio.
The Thai European Tour Member opened with three birdies in a row, starting from the tenth tee on the tough, 7,400 yard, par 70 South Course, but closed bogey, bogey on the way to a seven birdie, three bogey 66.
That gave him the early clubhouse lead at four under par, where he was joined by Clark, whose 66 featured six birdies and two bogeys.
A 67 from American Steve Stricker left him a shot behind Marksaeng and Clark while there were 68s from US Ryder Cup star Hunter Mahan, Australian Robert Allenby and Spain's Miguel Angel Jiménez.
Open Championship winner Stewart Cink and England's Ian Poulter were among a group of late starters quickly moving to two under.
Cink, playing for the first time since defeating Tom Watson in a play-off at Turnberry last month to claim his first Major victory, birdied the first two holes and was two under after six.
Poulter, meanwhile, birdied the second and third holes and was two under after four.
US Open Championship winner Lucas Glover, Players Championship winner Henrik Stenson and two time US PGA Tour winner this year Kenny Perry were among those in the clubhouse with 69s while Lee Westwood, a joint runner-up here last year, six time champion Tiger Woods and US PGA champion Padraig Harrington all reached one under early in their rounds.
Defending champion Vijay Singh, World Number Two Phil Mickelson and Masters Tournament champion Angel Cabrera were in a large group that posted level par 70s.
Mickelson, returning to golf after taking six weeks off to care for his wife Amy as she began treatment for breast cancer, showed some rust with a double-bogey six at his second hole of the day, the 11th, and went out in a three over 38 only to claw his way back with three birdies on the back nine.
Also at level par were 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, Eve Rose, who was born the Saturday after he had contended on the final day of last month's Open Championship.
“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.
“It was a decent start but it was a bit frustrating. I felt like I played really good for my front nine but just couldn’t get anything going on the greens, didn’t hole anything.
“Then I hooked a drive up the first almost in the water and hit a shocking three wood into the second and I walked off with a par and a birdie. So it was a little bit of a strange day to be honest but level par is a decent start.”
Poulter was getting his challenge off to a great start, adding back to back birdies at the tenth and 11th to move to four under par after 12 holes, and on the same mark as the clubhouse leaders.
Westwood had got to three under to go out in 32 before bogeys at the 11th and 13th sent him back down the leaderboard.
Harrington passed the Englishman going in the opposite direction, going out in 33 thanks to birdies at the first and seventh and adding another at the tenth to reach three under, alongside Cink and New Zealand 19 year old Danny Lee.
Woods, meanwhile, returned to level par after 11 holes having bogeyed the eighth.
Paul Casey, England’s World Number Three and The Race to Dubai leader, was forced to withdraw because of an injury after just six holes.