Novak Djokovic double-faulted, then shook his right arm and grimaced.

Seconds later Monday night, a weak serve produced a wince from the U.S. Open’s defending champion, then was followed by a missed forehand that gave away a set — the first set dropped by Djokovic in the first round of any Grand Slam tournament since 2010.

While he managed to emerge with a 6-3, 5-7, 6-2, 6-1 victory over Jerzy Janowicz of Poland, there were plenty of signs of trouble, starting with a visit from a trainer who massaged Djokovic’s bothersome arm after only five games.

Asked about his health during an on-court interview, Djokovic deflected the question, saying, “I don’t think it’s necessary to talk about this now. I’m through. I’m taking it day by day.”