Britain's Graeme McDowell prepared for an early morning tee-off to launch his title defence as the 111th U.S. Open began under overcast skies at Congressional Country Club on Thursday.
McDowell, who claimed his first major title by one shot at Pebble Beach last year, was scheduled to set off on the first hole at 0755 local (1155 GMT).
The Northern Irishman has been drawn with British Open champion Louis Oosthuizen of South Africa and American Peter Uihlein, the reigning U.S. amateur winner, for the first two rounds.
World number one Luke Donald has been grouped with second-ranked fellow Briton Lee Westwood and third-ranked German Martin Kaymer with the high-profile trio scheduled to tee off on the 10th hole at 0806 (1206).
American Phil Mickelson, who has finished runner-up at the U.S. Open on five occasions but never won the title, was among the afternoon starters in a glamour pairing with big-hitting compatriot Dustin Johnson and Britain's Rory McIlroy.
Johnson and McIlroy have both been in contention to win majors in recent months but squandered 54-hole leads going into the final round to come up short.
With former world number one Tiger Woods a notable absentee this week while he continues to recover from his left knee and Achilles tendon injury, the year's second major is wide open for the taking.
Kaymer, who landed his maiden major crown at last year's PGA Championship, believes up to 40 players in the 156-strong field are genuine contenders for the title.
WIDE OPEN CHAMPIONSHIP
"Probably 10, 15 years ago there were only 10 or 12 players who could win but now it's so spread out," the softly-spoken German told reporters.
"We have had 10 different winners in the last 10 majors that we've played so it's very open at the moment. It can be anybody this week -- 30 to 40 players is my guess."
Seven of the last eight major champions have been first-time winners and neither top-ranked Donald nor his fellow Englishman Westwood has yet lifted a grand slam title.
In other first-round groupings on Thursday, Sergio Garcia has been paired with fellow Spaniards Miguel Angel Jimenez and Alvaro Quiros while the Italian brothers Edoardo and Francesco Molinari will be joined by their countryman Matteo Manassero.
Masters champion Charl Schwartzel of South Africa was set to tee off in the afternoon with two other winners of the green jacket, Trevor Immelman, also of South Africa, and Zach Johnson.
The first blow of the 2011 U.S. Open was struck by South Korea's Kim Dae-hyun who pushed his tee shot into the right fairway bunker at the par-four opening hole.
American Chez Reavie split the fairway with his drive while Ireland's Shane Lowry also ended up in the right bunker.
By the time the players prepared to hit their second shots, light rain was falling at Congressional. A mixture of sunshine, showers and thunderstorms is forecast for the rest of the day.