PGA Tour rookie Shawn Stefani fired a six-under-par 65 to lead by two shots at the end of day one of the Tampa Bay Championship.

Starting at the 10th, the 31-year-old birdied his first two holes, then picked up another four shots along the way in a blemish-free round to lead by two from fellow American Brian Harman.

One shot back on three under was another trio of Americans, Tag Ridings, Brendan Steele and Harris English.

Despite having such a good day, Stefani still was not entirely satisfied with his performance.

"I kind of fought through a couple of bad shots - or not-so-great shots - and managed to get balls up-and-down and make a few putts," he said.

"So all in all, today was a great day and I'm looking forward to getting out there tomorrow."

England's Luke Donald, the defending champion, was the leading European, in a share of 14 at one under par, but it could have been better.

Donald had three bogeys in his closing four holes in a hugely frustrating end to his opening round.

Donald began on the back nine and was building a platform from which to make a serious tilt for another victory until the round unravelled.

From a position where he led on four under par, England's former world number one slumped back to one under and into the pack. Fives at the par-four sixth and ninth, and a four at the short eighth, did the damage.

On his final hole, Donald found rough off the tee, planted his second shot into a bunker, and even then could not find the green. He at least put the ball within five feet from his fourth shot, and made the bogey putt.

Donald's compatriots Brian Davis and David Skinns were level par with Greg Owen one stroke further back, whuile Ross Fisher had a day to forget with a seven-over 78 which included three double-bogeys.

Scotland's Martin Laird was two over.