World No1 Dustin Johnson emerged as a doubt on the eve of the Masters after he hurt his back in what his agent described as "a serious fall".
Johnson fell on the stairs of his rental home in Augusta, where the year's first Major is due to get under way today.
He received medical attention and was advised to rest in the hope of recovering sufficiently to tee off and bid for a first Green Jacket.
"Dustin took a serious fall on a staircase in his Augusta rental home," his representative David Winkle said on Wednesday night.
"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."
Johnson had enjoyed a flawless build-up to the Masters, winning three tournaments in a row to establish a sizeable led at the top of the world rankings.
The US Open champion's dominance and athleticism have earned comparisons with the glory days of former world No1 Tiger Woods.
Should he be passed fit, Johnson is due to begin his round shortly after 7pm British time.
He is due to tee off in an all-American group alongside two-time Masters winner Bubba Watson and current US PGA champion Jimmy Walker.
His absence would leave Northern Ireland's Rory McIlroy, who needs an Augusta triumph to complete a career grand slam of all four Majors, and 2015 winner Jordan Spieth as the bookmakers' favourites.
Johnson's most recent title came at last month's WGC Match Play event. He never trailed in any of his seven matches and beat Spanish rising star Jon Rahm in the final.
His other wins this year came at the WGC Mexico tournament and the Genesis Open.
Masters organisers, meanwhile, were forced to abandon the traditional par three contest for the first time in its history due to bad weather.