Defending US Open champion Novak Djokovic complained of feeling "quite rough and unpredictable" as a nagging shoulder problem hampered him during Wednesday's second-round win over Juan Ignacio Londero.
The world number one received medical treatment to his left shoulder throughout the match on his way to a 6-4, 7-6 (7/3), 6-1 win and said he feared the injury could have forced him to retire.
"It was not easy to play with this kind of sensation, to be honest. I did not experience that too many times in my career," Djokovic said.
"I had obviously, you saw, a medical timeout. At changeovers, I tried to use within the rules as much as I can physiotherapy and medical help. That has definitely helped me stay in the match.
"The way it has started for me, especially midway through the first set, I didn't know if I would be able to finish the match."
Djokovic, who is attempting to become the first back-to-back men's champion here since Roger Federer won from 2004-2008, said the problem affected both his serve and backhand. He also revealed it was an issue he has been dealing with for "quite a while".
"It is new in a sense that I've never had that particular issue in my career. It's not new in a sense that it has bothered me now for almost a couple weeks," he said.
"I've been experiencing some days of higher intensity of pain, some days less. It has been really fluctuating a lot, going up and down. What happened today on the court, actually how I felt, was quite rough and unpredictable."
Crucial day off
Top seed Djokovic will play the winner between 27th seed Dusan Lajovic and American Denis Kudla in the third round on Friday and suggested he would "freeze" his arm for the next 48 hours in the hopes of recovering.
"Good thing about Grand Slams is you have a day off in between the matches. As I said, I'm hoping that with a proper medical help and treatments, I'll be able to get myself in a better state than I was today in few days," Djokovic said.
"I'm going to assess this injury tomorrow even more with further consultations with experts in sports medicine. I'm hoping that in two days' time I will be able to play pain-free, if that is possible."
Djokovic has won four of the past five majors, and 16 overall, but was tested by 56th-ranked Londero, who led 3-0 in the second set before the Serb stormed back by winning the next five games.
Londero then broke back, held at love and forced a tie-breaker, but Djokovic won five of the last six points thanks to his opponent's errors and went on to dominate the final set.
"It wasn't easy to play with the pain, but you have to hope you will get some opportunities and some lucky shots."
"I was also lucky to find my way back in the second set and to win in straight sets," Djokovic added.
"It's not the first time I'm facing this kind of adversity or challenge. It is what it is and I'm just grateful to be on the court."