NEW YORK (AP) — Top-seeded Novak Djokovic showed no signs of any trouble with his right shoulder Tuesday, building a 6-0, 5-1 lead in the first round of the U.S. Open before his ailing opponent stopped playing.

Djokovic won the first seven games, then had reeled off the last 16 points in a row when qualifier Conor Niland of Ireland retired. Niland had been seen by a trainer and was bent over, hands on knees, after several points.

It was Djokovic’s first match since he quit because of a painful shoulder while trailing in the second set of the Cincinnati Masters final on Aug. 21. That went into the books as the Serb’s second loss of the year.

The result against Niland improved Djokovic to 58-2 with nine titles in 2011, including at Wimbledon and the Australian Open.

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