World number two Lee Westwood made an encouraging start to the Houston Open by shooting a four under par 68 in the last event before the Masters.
The Englishman and Ireland's Padraig Harrington were in a group five shots behind leader Jimmy Walker, who carded a flawless nine-under par 63.
The American matched the course record to keep him two shots ahead of chasers Nick O'Hern and Josh Teater.
Reigning Masters champion Phil Mickelson hit a two-under par 70.
Returning from a three-week break, Westwood hit five birdies on the back nine to put him back in contention after the first round.
If the Nottinghamshire golfer can finish in the top two at the Redstone Golf Club this weekend he will climb above Germany's Martin Kaymer at the top of the World Rankings.
Italy's Francesco Molinari finished the opening day on three under, while England's Ross Fisher and Brian Davis shot rounds of 71, as did German Alex Cejka and South Africa's former Masters runner-up Ernie Els.
European Ryder Cup captain and former Masters champion Jose Maria Olazabal had a day to forget after carding a 77.