World number one Dustin Johnson could miss the Masters after injuring his lower back in a "serious" fall on the eve of the tournament.
The 32-year-old American star landed "very hard" after a fall down the stairs at his rented house in Augusta on Wednesday, his agent said, but still hopes to be fit enough to tee off in the first round.
Johnson is the oddsmakers favourite to collect the winner's green jacket at Augusta National after winning his past three starts.
"At roughly 3 pm today, Dustin took a serious fall on a staircase in his Augusta rental home," said his agent, David Winkle, 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."
Johnson is scheduled to tee off in Thursday's final first-round group at 2:03 pm alongside compatriots Jimmy Walker and Bubba Watson.
Even if he is able to start, his trademark long driving skills could be compromised if the back trouble continues, a potentially serious handicap over the 7,435-yard Augusta National layout.
Johnson averaged a tournament-best 299.5 yards in driving distance last year, when he finished a career-best fourth, and averaged a Masters-low 26.50 putts per round in 2015, when he shared sixth.
"Everything has got to be working this week," Johnson said Tuesday. "If you want to win, you're going to have to drive it well. You're going to have to hit your irons well.
You're going to have to putt it well.
"I don't know how many guys are in the field, but they are the best golfers in the world, so you've got to bring your best stuff if you want to win."
The injury marks another setback in the checkered career of Johnson, who won his first major title in 2016 at Oakmont.
Johnson withdrew from the 2012 Masters after a back injury suffered, his agent said, when he lifted a jet-ski.
At the 2010 PGA championship, Johnson carried a one-stroke lead into the final hole but received a two-stroke penalty for grounding his club in a bunker, doing so in a sandy area he did not recognise as a hazard.
Even in his major victory last June, Johnson had to play the final holes knowing he would likely receive a penalty over an earlier infraction after a rules official had told him to play on.
Johnson became the world number one by winning at Riviera in February and followed with World Golf Championships Mexico and Match Play titles in March. He would be the first player since 1976 to enter the Masters on a three-event win streak.
Johnson — born an hour's drive north of Augusta National in Colombia, South Carolina — played 112 holes and never trailed in a single match of his Match Play triumph, which made him the first player to win all four WGC events.
The last time a world number one missed a major tournament was in 2015 when Rory McIlroy suffered an ankle injury a few weeks before he was to have defended his title at the British Open.
Johnson has not missed a major since the 2014 PGA Championship, which came just after he announced he was taking a six-month break from golf to seek professional help for "personal challenges."
No world number one has won the Masters title since Tiger Woods in 2002 and no player since 1940 has made the Masters his fourth victory in a row.
Johnson hopes to make his seventh Masters start after unleashing a late charge on Sunday last year. He made birdies at the par-5 13th and 15th holes but a double bogey at 17 ended his chances to overtake eventual winner Danny Willett of England.