Jordan Spieth had a bogey-free first day to sit two strokes ahead of the field with Sergio Garcia on seven-under 63 at the Charles Schwab Challenge, but Graeme McDowell is well down the field in Texas.
McDowell posted three birdies in the last six holes to eventually card a 74. That sees him back at four ender, in a share of 98th place.
Spieth, who won at Colonial Country Club in 2016, putted strongly to gain more than four strokes on the greens and missed just one putt from 18 faced inside 20 feet.
Spain's Garcia, who snared his first PGA Tour victory at Colonial 20 years ago, had a trio of birdies from the eight to the 10th holes before he eagled from a bunker on the par-five 11th.
Americans Erik Compton and Jason Kokrak each carded 65 while their compatriots Brandt Snedeker and Kramer Hickok were another stroke back with Canadian Adam Hadwin.
England's Ian Poulter and Justin Rose were five strokes off the pace, with Poulter carding three birdies and a bogey while Rose mixed four birdies with two bogeys.
Fellow Englishman Matt Wallace was eight strokes behind Spieth and Garcia.
Newly-crowned US PGA champion Phil Mickelson had a poor start in his attempt to push for an unlikely Ryder Cup place, with the 50-year-old recording a 73 after five bogeys to sit 10 strokes back along with Scotland's Russell Knox.