New York: 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."
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.