Westwood shot a 5-under 67 at the Augusta National Golf Club, finishing his round late in the afternoon just as the forecast showers began to dampen the course. The rain did not stop play, and the first round of the year's first major championship was completed with 28 players within four shots off the lead.
The anticipated duel between Tiger Woods and defending U.S. Open champion Rory McIlroy was put on hold for at least another day, Phil Mickelson lost a ball and perhaps his chance of a title as well, and a collection of players saw promising beginnings disappear down the stretch.
Westwood put himself in position for his first major title with four straight birdies beginning at the par-4 fifth. He had seven birdies overall on a course that proved difficult to conquer.
Former British Open champion Louis Oosthuizen and Peter Hanson shared second place at 68 while the group at 69 included Ben Crane, Francesco Molinari, Miguel Angel Jimenez, Paul Lawrie, Bubba Watson and last year's PGA Championship runner-up Jason Dufner.
Jim Furyk and two-time Masters champion Vijay Singh were among those at 70 and the large group at 71 included amateur Patrick Cantlay, former Masters winner Angel Cabrera, Keegan Bradley and McIlroy, who double bogeyed the opening hole and birdied the last two to break par.
Woods, who in his most recent outing won his first PGA Tour event in more than two years, bogeyed the final two for an even-par 72. He drove into the trees on the first two holes, survived with a par at both and then held his round together until the struggling finish. Defending champion Charl Schwartzel also shot 72.
World No. 1 Luke Donald stumbled to a 75 and Mickelson had a 74 that included a triple-bogey seven at the 10th after his tee shot disappeared in tall grass.
Woods was not alone among those who let a good score get away.
Hunter Mahan, the only two-time winner on the tour this year, was 3-under through five holes but wound up with a 72. Veteran Scott Verplank played the first eight holes in 4-under, but finished with a 73.
Former Players Championship winner Henrik Stenson suffered most of all. He shot a 5-under 31 on the front nine, eventually got to 6-under and went to the 18th tee needing a par to shoot a 67. Instead, he drove into the trees, took two shots to hack it into the fairway and then found his ball in a terrible lie.
After chopping the ball up the fairway, Stenson slammed his club into the turf in anger -- a sight seldom seen at a tournament where etiquette is appreciated as much as winning golf. He finished with an 8 on the hole and walked away with a 71.