Phil Mickelson took a share of the lead on the opening day of the Deutsche Bank Championship after a sublime eight-under-par 63 - yet it could have been even better for the reigning Open champion.

Mickelson, fourth in the FedEx Cup standings, was well on course to card a magical 59 after starting his round on the back nine with seven birdies to reach the turn on 28.

Yet the front nine posed a different proposition for the left-hander, who found the bunker at the first for his only bogey of the round before bouncing back with an eagle on the next after landing his second shot to two feet from the pin.

However, a run of pars followed before the American finished his round birdie-bogey at TPC in Boston to share the lead with England's Brian Davis.

Defending champion Rory McIlroy was left frustrated after a topsy-turvy one-under-par 70, with Graeme McDowell two shots further back.

McIlroy has endured largely frustrating fortunes this year and on Friday he recorded six birdies and five bogeys.

"The mistakes are coming from missing it in the wrong spots," he said.

"It's a bit mental but I still made six birdies, so not that much is wrong, (but) it's the worst it could have been."

Phil Mickelson obviously reflected on what might have been.

"I still felt I could have shot three or four under (on the finishing nine),".I wasn't too worried about shooting 59," he told the PGA Tour website

After opening with a round of 60 at the Waste Management Phoenix Open earlier this year, the 43-year-old was satisfied with his round, however.

"I putted really well. There were a couple that didn't go that had every bit as good a chance to go as the ones that did and that's the sign that you're putting well," he added.

"There was only one putt the entire I should have made; the par putt on one that didn't go in and the other ones were really good."

Davis, 80th in the FedEx Cup and needing a good week to qualify for the BMW Championship for the fifth time in his career, joined Mickelson at the top of the leaderboard after an unblemished round.

The 39-year-old catapulted up the standings with five birdies in seven holes before finishing with back-to-back birdies to move to eight under, one shot ahead of Kevin Stadler, who made eight birdies and a solitary bogey on the 12th in his round of 64.

World number one Tiger Woods and Barclays winner and Masters champion Adam Scott were in the same playing group as Mickelson although they failed to take inspiration from the 43-year-old.

Woods, who on Thursday told reporters his back problem is not as bad as first feared, struck a 68 although Scott was well off the pace and is tied for 87th following a two-over-par round.

American pair Hunter Mahan and Roberto Castro, and Sergio Garcia were on six under par, while Englishmen Lee Westwood and Ian Poulter were among 16 players on shot further back.