Dustin Johnson, the reigning US Open champion, had been the heavy favorite to win the year's first major golf championship, but suffered a lower back injury on Wednesday in a fall down stairs at a rental house.

Johnson was seen walking back to the clubhouse shaking his head after undergoing a rigorous warm-up session before admitting defeat.

The 24-hour drama reached its peak when Johnson arrived at the course and practiced his swings, hoping he could summon the strength to play through pain.

After a physical therapy session, Johnson tested his swing for several minutes under the watchful gaze of his caddie, brother Austin Johnson, and swing coach Butch Harmon, then returned for more therapy.

Johnson was due to have joined two-time Masters champion Bubba Watson and 2016 PGA Championship winner Jimmy Walker in the day's final group.

Not since Rory McIlroy withdrew from the 2015 British Open has a world number one not played in a major championship. The only other top-ranked player to miss the Masters was Tiger Woods, who did not play in 2014 after back surgery.

Hoffman takes early lead

The show must go on in Augusta, with or without the world number one. Just 11 players managed to break par on Thursday in the face of strong winds gusting up to 40 mph (64kph) on the course. 

Charley Hoffman leads the way with a 7-under 65. Hoffman blistered through the back nine, with five birdies to take a four-stroke lead over William McGirt. That is the largest opening round lead at the Masters since 1955. Lee Westwood is in third with a minus-2 score. Three-time Masters champion Phil Mickelson, Sergio Garcia and Justin Rose are tied with five others for fourth at one under par. 

Defending champion Danny Willett shot a 73 while 2015 champion Jordan Spieth hit a quadruple bogey on 15 and finished Thursday with a 75, tied with Germany's Bernhard Langer. German Martin Kaymer finished the day shooting 78.

mds, kbd/kl (AFP, AP)