Pre-tournament favourite Dustin Johnson has suffered a lower-back injury following a fall at his rental home ahead of Thursday's opening round of the Masters in Augusta.
His agent David Winkle says he still hopes to play tomorrow.
World number one Johnson fell on the stairs on Wednesday and "landed hard on his lower back".
He is said to be uncomfortable but is resting and doctors have advised him to remain stable.
Johnson is due to tee off in the last group at 19:03 BST on Thursday evening.
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"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."
The American, 32, won his third successive tournament when he beat Spain's Jon Rahm in the World Match Play final in late March.
He has won seven of the 17 tournaments he has played since claiming his first major at the US Open at Oakmont in June, racking up another seven top-10 finishes in the process.
Analysis
BBC Sport's Iain Carter:
There is the adage of "beware the injured golfer" but there is no doubt this is a significant blow for Johnson.
Since winning last year's US Open he has been a commanding presence and built a telling aura.
Now his Masters bid is surrounded by uncertainty. His late tee time may prove a blessing as it gives an extra recovery period but there is no doubt this is a considerable setback.
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