NEW YORK – 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.”

When the subject arose at his news conference, Djokovic again avoided addressing the topic, saying the trainer’s visit “was just prevention; it’s all good.”

During the match, Djokovic hit first serves around 100 mph, sometimes slower – 25 mph or so below what’s normal for him. He hit second serves in the low 80s mph. He flexed that right arm, the one he has used to wield a racket on the way to 12 Grand Slam titles, and appeared generally unhappy, covering his head with a white towel at changeovers.

Djokovic’s coach, Boris Becker, gnawed on his fingernails, looking nervous as can be.

All in all, Djokovic’s issues figure to loom large as the tournament progresses, and therefore were the most noteworthy development on a Day 1 at Flushing Meadows that did include drama elsewhere.

There was 20th-seeded John Isner’s comeback from two sets down to edge 18-year-old Frances Tiafoe before a rowdy, standing-room-only crowd at the new Grandstand. And 26th-seeded Jack Sock’s five-set victory over 18-year-old Taylor Fritz in another all-American matchup.

More, too: A first-round loss by Rio Olympics gold medalist Monica Puig, and French Open champion Garbine Muguruza’s complaints about having trouble breathing after dropping the first set of a match she would go on to win in three.

This was the No. 1-ranked Djokovic’s first match at a major since losing to Sam Querrey in the third round of Wimbledon, which ended the Serb’s bid for a calendar-year Grand Slam after titles at the Australian Open and French Open. He exited the Rio Olympics in the first round this month.