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 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."

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.

In the stands, Djokovic's coach, Boris Becker, gnawed on his fingernails, looking nervous as can be.

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, Jack Sock withstood Taylor Fritz's repeated comebacks to win the second five-set match between American men Monday at the U.S. Open.

The 26th-seeded Sock took the first two sets, dropped the next two, then went up two breaks in the fifth only for the 18-year-old Fritz to erase both of those. But Sock broke Fritz's serve again to clinch a 7-6 (3), 7-5, 3-6, 1-6, 6-4 win in 3 hours, 21 minutes.

Sock overcame 17 double-faults and 73 unforced errors to beat Fritz in five sets in the first round at a major for the second time this year — they also met at the Australian Open in Fritz's Grand Slam debut. The players combined for 99 winners Monday.

Fritz, last year's U.S. Open boys singles champ, is ranked a career-best 53rd.

Earlier Monday, 20th-seeded John Isner beat another 18-year-old, Frances Tiafoe, in five sets in an all-U.S. matchup. Gael Monfils and Marin Cilic were also winners.