* World number one doubtful for U.S. Masters
* Also missed 2012 Masters with back injury (Adds details)
AUGUSTA, Georgia, April 5 (Reuters) - World number one Dustin Johnson's participation in the U.S. Masters this week is in doubt after he injured his back in a freak accident, his manager David Winkle said on Wednesday.
Johnson, the U.S. Open champion, is favourite to win the Masters title after victories in his last three tournaments.
"At roughly 3:00 pm today, Dustin took a serious fall on a staircase in his Augusta rental home," Winkle said in a statement.
"He landed very hard on his lower back and is now resting, although quite uncomfortably. He has been advised to remain immobile and begin a regimen of anti-inflammatory medication and icing, with the hope of being able to play tomorrow. I will refrain from commenting further until I know more about Dustin's condition."
No world number one has captured the Masters Green Jacket since Tiger Woods in 2002.
"It's a funny game. It doesn't matter how good you're playing, you can still not win," Johnson, who became world number one in February, said on Tuesday.
"I've got a lot of confidence in my game right now, especially with the way I've been playing the last few tournaments. But, you know, anything can happen."
Apart from top-10 finishes the last two years, Johnson has a modest record at the Masters.
His dreams of winning last year were ruined by two double-bogeys in the final round. He tied for fourth, four shots behind winner Danny Willett.
It is not the first time Johnson's bid for a Green Jacket has been hit by injury.
The big-hitting American was forced out of the 2012 Master after injuring his back while lifting a jet ski. (Editing by Steve Keating)