Goosen has been working hard to improve fitness and form

South African Retief Goosen took the lead after the first round of the WGC-Bridgestone Invitational in Ohio.

Goosen shot five birdies and one bogey in a four-under-par 66 to lead by one stroke from Tim Clark, Daniel Chopra, Vijay Singh and Zach Johnson.

Steve Webster is the highest placed Briton at two under while Open champion Padraig Harrington is one under.

Open runner-up Ian Poulter finished his opening round on an even par 70 at the Firestone Country Club.

"It was steady, I drove fantastic, hit every fairway with a driver and once with a three wood and just played really, really rock solid," said Poulter.

I just struggled, I wasn't with it at times Padraig Harrington after carding a round of 69

"It was a shame to bogey the last. I hit the seven-iron very heavy but I'm still alive, it's all good."

Double major champion Goosen has not won in America since 2005 but believes he is returning to his best form.

"The last four months I've been working a bit more on my fitness," said Goosen. "I've lost 18 pounds in weight since the beginning of the year.

"So I'm definitely fitter and a bit stronger than I was last year. So I'm working a little bit more on that.

"My swing, me and golf instructor Gregor Jameson are working on it, we're slowly getting it to where we want it to be, just a matter of getting out there and playing.

"Everything is starting to fall together. My putting is a little bit better, I think, as well."

Singh was five under down the last but blotted his copy book with a double bogey.

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Harrington got off to a poor start and was two over after 13 holes but recovered with three birdies on the final five.

"I just struggled," said Harrington. "You know, I wasn't with it at times.

"I didn't do any harm today, which is great for me, and if I can get better each day, that would be great."

Colin Montgomerie, desperate for Ryder Cup qualifying points, finished two over.