-World number one doubtful for US Masters

-Also missed 2012 Masters with back injury

World number one Dustin Johnson's participation in the US Masters this week is in doubt after he injured his back in a freak accident, his manager David Winkle said on Wednesday.

Johnson, the US 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."

The Masters starts in Augusta on Thursday with Johnson scheduled to play alongside fellow Americans Bubba Watson and Jimmy Walker in the last group out.

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.