Dustin Johnson's participation in the Masters is in doubt after the world no 1 injured his back falling down stairs on the eve of the tournament.
Johnson, who was favourite to win the famous Green Jacket for the first time, still hopes to be able to play in the tournament and is having treatment for the injury.
"At roughly 3pm today, Dustin took a serious fall on a staircase in his Augusta rental home," his agent David 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."
Johnson is due to be the second-last player to tee off, at 2.03pm (7.03pm Irish) local time.
Johnson won the Genesis Open in February to reach world number one and then added victories in the WGC-Mexico Championship and WGC-Dell Technologies Match Play.
The 32-year-old has also finished sixth and fourth in his last two Masters appearances, but was taking nothing for granted in his pre-tournament press conference on Tuesday.
"I mean, golf, it's a funny game," the US Open champion said. "It doesn't matter how good you're playing, you can still not win. Same goes for this week.
"If I want to win here, everything's going to have to go well for me. I'm going to have to drive it well, hit my irons well, putt it well. Everything is going to have to be really good.
"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."
Johnson missed the 2014 Ryder Cup after taking a six-month leave of absence from the game to deal with "personal challenges", prompting allegations in the United States of a positive test for cocaine.
The following January, Johnson gave an interview to the same media outlet which had printed those claims and flatly denied the allegations, but admitted he had issues with excessive drinking.
He went on to explain that he had cleaned up his act as his fiancee Paulina Gretzky was expecting their first child, with the couple's son Tatum born that month, and the following year he claimed a first major title in the US Open after several heart-breaking near-misses.
"My quest with him was always to get more serious about it, to work harder at it away from the tournaments and during his time off," his coach Butch Harmon, said this week.
"You want all the guys to have a life, but DJ is a little bit of a partyer and didn't work as hard as he could because his natural talent carried him. I think he realised that and once we woke up a sleeping giant and he started working harder and harder, now he's starting to see the fruits of his labour.
"He continues to work hard and I must say as his coach I am very proud of what he's done both on and off the course.
"He has matured a lot in every part of his life. DJ is a free spirit, he likes to have a good time, but he really has dedicated himself this year.
"His goal at the start of the year was to be (world) number one. I told him I thought he'd be number one by the Masters, at the latest by the US Open, and he got there sooner than that."