World number one Novak Djokovic was confident he would be fit enough to play in the U.S. Open despite a worsening shoulder problem that forced him to quit Sunday's Cincinnati Open final against Andy Murray.
Djokovic was looming as the overwhelming favourite to win the last grand slam of the year, starting in New York on Aug. 29, but his preparations were thrown into disarray because of the niggling injury.
Djokovic was in clear discomfort in Sunday's final against Murray. The Serbian called for a medical time-out after losing the opening set 6-4 then threw in the towell when he fell 3-0 behind in the second set.
His withdrawal immediately raised concerns about his prospects at the U.S. Open but he later said it was just a precaution and he expected to play at Flushing Meadows after a week of rest.
"I am confident that I can recover and be ready for the U.S. Open," he told reporters.
"It's unfortunate that I had to finish this way.
"I apologize to the tournament. I apologize to the people who came here today to watch the match. I really tried (but) it didn't make sense for me to continue."
Djokovic has been heavily criticized in the past for quitting matches but the 24-year-old said it made no sense to continue against Murray.
He service speed was below his normal standards and his groundstrokes were erratic and lacking power. In the first set alone, he made more than 20 unforced errors.
"The reason (I retired) is shoulder pain. I just could not serve. I served an average 90 miles per hour the first serve, and I could not play forehands," he said.
"I could have maybe played another couple of games, but what for? I cannot beat a player like Murray today with one stroke."
Djokovic, whose only previous defeat this year was to Roger Federer in the semi-finals of the French Open, won last week's Masters event in Montreal and decided to back up in Cincinnati even though his body was already showing signs of wear and tear.
"It has been there for about ten days," he said.
"It was increasing, the pain was increasing, but we were trying to maintain the good condition. Today was just too much."
The U.S. hardcourt season has always been an extreme test of physical strength and Djokovic, who has won 57 matches and two grand slam titles this year, said he was not surprised that he had finally succumbed to an injury. "It's kind of expected. I've played so many matches this year," he said.
"I've been winning, you know, a lot and reaching the final stages of each event that I've participated on.
"Considering the schedule that is very busy in tennis, it's kind of normal to expect that at some stage you are exhausted."