Australian Adam Scott and Hunter Haas of the United States shot opening-round 66s to co-lead the AT&T National in Pennsylvania by one stroke.
Scott and Haas each carded five birdies to end the day on four under, one clear of a group including Jhonattan Vegas.
Justin Leonard and Vijay Singh are in a group of 10 players on two under, two strokes off the first-round leaders.
England's Justin Rose posted an opening 70 with compatriot Brian Davis one stroke further back.
Players found the going tough at the testing Aronimink course, with only 28 of the 120 players breaking par.
Scott holed birdie putts of 20 feet and 25 feet early in his round and had a 15-foot par save on the 17th and another 15-footer for birdie on the 18th. His only bogey came on a three-putt from the fringe on the 10th.
Afterwards Scott, who finished joint second at this year's Masters, said his driving was the important factor in his early charge.
"I think the key to my round was hitting a lot of fairways, which gave me an opportunity to hit the greens," said the 30-year-old, after equalling his best round on the PGA Tour this year.
"The few fairways that I did miss, I didn't get into too much trouble. It was kind of a stress-free round, which is a nice way to start.
"It's a great golf course, obviously, but in great shape. But they've set the course up beautifully. The greens are perfect and they've got very generous fairways, but severe rough, which is a nice balance. There are good scores out there, but you have to play well."
Haas produced four straight birdies on the back nine early in his round and he was the only player to get to five under until a bogey at the 7th when he got out of position off the tee.