Novak Djokovic's remarkable winning streak ended with a grimace, one that puts a new spin on the U.S. Open.

The world's top-ranked player was forced to retire in the second set Sunday because of a sore right shoulder, giving Andy Murray the championship at the Western & Southern Open.

It was the Serb's first bad moment in his 57-2 season. Djokovic had won 16 consecutive matches since his only previous loss of the season in the semifinals against Roger Federer on June 3 at Roland Garros. The winning streak has taken a toll.

Djokovic talked about feeling exhausted in Cincinnati, coming off his record fifth Masters series title in Montreal last week. He said his serving shoulder had bothered him for about the last 10 days, but he'd been able to manage the soreness and keep winning.

On Sunday, he couldn't go on.

"There is no good loss, that's for sure," Djokovic. "The good thing is there's a week, eight days to the start of the Open.

"I'm confident I can recover and be ready for the U.S. Open."

He was completely off his game against the fourth-seeded Murray, won the first set 6-4 and was ahead 3-0 in the second when Djokovic decided just before the rain came that he couldn't continue. Djokovic got his shoulder treated after he lost the first set, grimacing at one point.

With his serve registering only in double-digits and his forehand limited by the pain, Djokovic realized he couldn't compete. He said he would have retired even if the rain had temporarily stopped the match.

"I could have maybe played another couple of games, but what for?" he said. "I cannot beat a player like Murray today with one stroke."

It was Murray's second title this season. The 24-year-old Scotsman also won at the Queen's Club. He lost his other final match to Djokovic at the Australian Open.

Djokovic felt worn-down heading into the final, the strain of all those recent matches catching up with him. He said his loathing for losing was pulling him through matches.

It wasn't enough on Sunday.

Djokovic was off his game at the outset, repeatedly hitting forehands, backhands and volleys everywhere but in the court. Murray broke his serve to open the match and won 10 of the first 12 points.

Could Djokovic win another one while running on fumes and playing with a bum shoulder?

By the end of the first set, he knew the answer.

"I was generally exhausted playing many matches, but the exhaustion is not the reason," Djokovic said. "The reason is shoulder pain. I just could not serve."

Murray was more rested, though not by choice. He lost in the opening round at Montreal a week ago, giving him unwanted time to relax and work on his game. The break seemed to help — he didn't lose a set all week in Cincinnati.

"I had five or six days to get ready here," said Murray, who won his first Masters series championship in Cincinnati in 2008. "I've always played well on this court."

With the crowd behind him, Djokovic got back into the match by breaking Murray to tie the first set at 3. Murray broke him right back, ending a long baseline rally by coming to the net for a put-away volley. Djokovic walked around the court with a blank expression, his mouth open.

His demeanor said he was in trouble.

Djokovic made 20 unforced errors in the first set, which ended when he dumped a routine forehand into the net. Djokovic squatted in disappointment, then went to his chair and took a timeout to have a trainer stretch his right arm and shoulder.

When the trainer pushed on the side of his rotator cuff with his thumb, Djokovic grimaced.

Murray broke him again to start the second set, an indication the shoulder wasn't going to get better. Djokovic started protecting the shoulder on his follow through.

When Murray left him with an easy volley into an open court, Djokovic couldn't get anything on his overhead return, smacking it weakly toward the net. That point gave Murray a 3-0 lead and forced Djokovic to accept that he wasn't going to get a chance to keep his winning streak going.

The health of his shoulder will become an overriding question heading into the U.S. Open, which starts on Aug. 29. Rafael Nadal is the defending champion, and he's also got some physical issues — burned fingertips on his right hand that bothered him in Cincinnati.