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

Seconds later Monday night, a weak serve produced a wince, then was followed by a missed forehand that gave away a set in the defending champion’s first-round match at the U.S. Open.

While he managed to emerge with a 6-3, 5-7, 6-2, 6-1 victory against 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.”

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.

This was the No. 1-ranked Djokovic’s first match at a major tournament 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.

Heading into the U.S. Open, Djokovic spoke about dealing with a left wrist injury that flared up in the days before the Rio Olympics this month. But that appeared to be just fine against Janowicz, a former top-20 player who reached the semifinals at Wimbledon in 2013 and is now ranked 247th after his own series of injury issues.

Earlier in Arthur Ashe Stadium, another two-time U.S. Open champion, Rafael Nadal, stood near the net after winning his first Grand Slam match in three months – 6-1, 6-4, 6-2 against Denis Istomin – and unraveled the thick wrap of white tape protecting his all-important left wrist. He said he’s still not back to hitting his forehand the way he does when he’s at his best, but there was nothing that seemed to be as debilitating as what Djokovic went through.

All in all, Djokovic’s issues figure to loom large as the tournament progresses and was the biggest development on Day 1 that did include drama elsewhere.

There was 20th-seeded John Isner’s rally 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 win 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.