Grayson Murray took the early lead with a round of 67 in the first round of the Valero Texas Open.
On level par, Graeme McDowell is five shots behind the leader having recovered on the 18th with a bogey-bogey-birdie finish in San Antonio.
He is in a group at 72, which includes defending champion Kevin Chappell
The American ended day one five-under, putting him one shot ahead of compatriots Chesson Hadley, Billy Horschel, Ryan Moore, Keegan Bradley and Matt Atkins.
The 24-year-old shot three birdies on the front nine of the San Antonio course, only faltering on the par-three seventh hole with a double-bogey.
But Murray rallied to hole four more birdies on the back nine and take the top spot on the leaderboard.
Of the five players making up the chasing pack with a round of 68, only Horschel and Moore hit a bogey-free round on the AT&T Oaks Course.
Two-time Valero Texas Open winner Zach Johnson finished on two-under 70 after striking a 12ft eagle putt on the par-five 14th hole and three straight birdies on the last holes.
Sergio Garcia, returning to the tournament for the first time in eight years and after a disappointing title defence at the Masters, carded two-under 74, while Adam Scott, who is aiming to hold on to his position of world number 59 to make the US Open next month, finished on 75 after a failing to make a single birdie.