World No. 1 Novak Djokovic was confident he would be fit enough to play in the US Open despite a worsening shoulder problem that forced him to quit Sunday's Western & Southern Open final against Andy Murray in Mason, Ohio.

Novak Djokovic kisses the trophy after winning the final of men's singles against Rafael Nadal of Spain in 2011 Wimbledon tennis championships in London July 3, 2011. [Xinhua]

Djokovic was looming as the overwhelming favorite to win the last grand slam of the year, starting in New York on August 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 4-6 then threw in the towel when he fell 0-3 behind in the second set.

His withdrawal immediately raised concerns about his prospects at the US 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 US 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 semifinals 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 24-year-old Murray, who has never won a grand slam title, remains cautious about writing off his main rivals.

"The thing is, you never know," he told reporters.

"Those guys could end up making the semifinals of the US Open, and then you get a chance to see whether it is a chance. If all of them lose early, then, you know, obviously my chances would go up.

"But I'm sure, come the start of the US Open next Monday, all of them will be fine. I think each one of them will be playing great tennis, much better than they have played here."