* Murray wins key U.S. Open warm-up tournament
* Djokovic succumbs to shoulder injury
Britain's Andy Murray won the Cincinnati Open on Sunday when his opponent in the final, Serbia's world number one Novak Djokovic, retired injured.
Murray was already in control and leading 6-4 3-0 when Djokovic, who had received treatment to his right shoulder at the end of the first set, called it quits, raising doubts over his fitness ahead of the U.S. Open.
It was just the second defeat this year for the in-form Djokovic, whose victories this season included the Australian Open and Wimbledon championships, but could not have come at a worse time with the last grand slam of 2011 due to start in New York on Aug. 29.
"It's unfortunate that I had to finish this way. I apologise to the tournament and I apologise to the people who came here today to watch the match," said Djokovic.
"I really tried but it didn't make sense for me to continue," he said, adding that he expected to recover in time for the U.S. Open.
Djokovic, chasing an unprecedented sixth Masters Series title this season, had struggled from the outset with Murray breaking his first service game.
The Serb showed some discomfort with his shoulder but was still able to break back to 3-3 and bring the crowd to life, in anticipation of a real contest.
But Murray, looking confident and efficient, broke again straight away, finishing off a magnificent rally with a superb cross-court forehand winner.
Murray served out the set and Djokovic called for a medical time-out during the interval, receiving an extensive massage to his right shoulder and upper-arm.
He returned to the court but was in clear discomfort and after dropping his first two service games to trail 3-0, he conceded defeat and came to the net to shake hands with Murray.
It was the world number four's second title in Cincinnati after he also beat Djokovic in the 2008 final and his seventh overall Masters crown and a major boost to his preparations for Flushing Meadows, particularly after an early exit in the Montreal Masters.
"It was really a good week after I struggled last week. I didn't drop a set and beat some very good players from pretty much the first round onwards," he said.
"Obviously with Novak today, it was unfortunate the way it ended, but I thought the first set, after the first couple of games, was a good standard.
"I thought there were a lot of long rallies and good points. It's been good preparation for New York."