Australian Adam Scott and American Hunter Haas shared the lead after the first round of the AT&T National at Aronimink Golf Club in Pennsylvania.
The pair went round in four under par 66s to lead b a stroke from Venezuela's Jhonattan Vegas and Haas' compatriots Dean Wilson, Joe Ogilvie and Kyle Stanley.
Scott, seeking his eighth US PGA Tour win, fired three birdies over the front nine but dropped a shot after three-putting at the tenth before recovering with gains at the 14th and 18th.
Scott's 66 equals his best round on the US PGA Tour this year and the 30 year old said: "I think the key to my round was hitting a lot of fairways, which gave me an opportunity to hit the greens.
"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."
Starting at the tenth, Haas picked up successive birdies on the 13th, 14th, 15th and 16th to lead the way by one shot as he reached the turn.
He made another gain at the fourth - his 13th - but dropped a shot on the seventh to ensure he would only be the co-leader overnight.
"I just really focused on keeping the ball below the hole and getting the ball in play," said Haas, who enjoyed his best round of the year.
"The holes I didn't get it in play, I hit great shots out of the rough and ended up making birdie off one of them, so that kind of helps."
Justin Leonard and Vijay Singh were among ten players sitting two shots off the lead overnight, while K J Choi was among 12 players to shoot one under 69s.
