Phil Mickelson and England's Brian Davis shared the lead after day one of the Deutsche Bank Championship after superb eight-under-par rounds of 63.
Mickelson marched to the top of the leaderboard early on after making a quite breathtaking start to his challenge at TPC Boston.
The reigning Open champion, who started at the tenth hole, looked on course to card a magical 59 at the halfway point of his round as he reached the turn in just 28 shots.
While the left-hander, who opened with a round of 60 at the Waste Management Phoenix Open earlier this year, then bogeyed the first, he bounced back with an eagle at the next after putting his second shot to two feet.
However, a run of pars then followed before the American signed off with a birdie-bogey finish to post a target that looked unlikely to be matched.
"I still felt I could have shot three or four under on the front nine," he said. "I wasn't too worried about shooting 59.
"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.
"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."
Mickelson appeared certain to end the day with the outright lead, but late starter Davis had other ideas and drew level with the five-time major champion after making birdie at his final two holes.
Good start for Englishmen
Kevin Stadler posted a 64 to sit in second by the close, the 33-year-old from Nevada producing five birdies in his final six holes to surge into contention.
Hunter Mahan and Spain's Sergio Garcia are a further shot back at six-under, while Englismen Lee Westwood and Ian Poulter are both part of a bloated 15-man group at five-under.
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 which featured four birdies and a solitary bogey.
Meanwhile, Scott was well off the pace and finished the day way down in a tie for 88th after a two-over-par 73.