FIRST ROUND LEADERBOARD USA unless stated -8 Patrick Sheehan -6 DJ Trahan, Brian Bateman -5 Brandt Jobe, Kenny Perry, Kevin Na (Kor) -4 Ted Purdy, Rodney Pampling (Aus), Kirk Triplett, Stephen Leaney (Aus), Harrison Frazar Selected others: -2 Bernhard Langer (Ger), Justin Rose (Eng) +1 Phil Mickelson

Sheehan's round was his best on the PGA Tour, including 11 birdies, only two of which were less than 10 feet.

"I didn't leave a thing out there," he said. "I got everything out of that I possibly could have, and more."

Fellow Americans Brian Bateman and DJ Trahan lead the chasing pack, with Justin Rose among those six adrift.

Steve Flesch shot a nine-over 79, the highest first round by a defending champion in the history of the tournament, which has not seen a repeat winner for more than 50 years.

Sheehan's previous career best was a 64 in the second round of the Byron Nelson in 2003.

Five straight birdies from the fourth hole took him to seven under and within reach of the PGA Tour record of 59.

"At that point, I'm apologizing to my playing partners because it was so ridiculous they were all going in," he added.

A double bogey on the ninth, after driving into a bunker, slowed his progress, but Sheehan got the shots back with birdies on holes 10 and 11, and added two further birdies.