NEW YORK (AP) — 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 US 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 US 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 US 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 therefore were the most noteworthy development on 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.

Like Muguruza’s win, Nadal’s afternoon match in Ashe was played with the new $150 million retractable roof open under a blue sky, while offering some extra shade on a day when the temperature reached 90 degrees.

The good news for Nadal, he said afterward, is that the pain is gone from his wrist, which whips those violent, topspin-heavy forehands that are the key to his success _ 14 of his 21 winners came off that wing.

The bad news for Nadal?

He still is working on feeling comfortable hitting down-the-line forehands, in particular, after sitting out _ not just zero real matches, but barely any practice, either _ from his withdrawal at the French Open in late May to the Olympics this month.

“Not easy to go 2 1/2 months out of competition, in the middle of the season, without hitting a forehand,” Nadal said. “I need to have the confidence again with my wrist.”

Both Nadal and his coach, Uncle Toni, described the way Rafael changed the way he hits a forehand during the Rio Games to try to avoid pain.

Both said things are improving.

But as Toni noted: “We need a little time.”

Istomin, who is from Uzbekistan and is ranked 107th, was not exactly likely to give Nadal much of a test. He entered the day 0-4 against the two-time US Open champion, having dropped 10 of the 11 sets they’d played. He had lost his past 20 matches against top-10 opponents. Plus, he’s been dealing with an injured right hamstring lately.

So what did he think of Nadal’s play Monday?

“For the first set, I was feeling that he was not hitting hard,” Istomin said. “A lot of short balls.”

Nadal’s summation of his day: “Not very good; not very bad.”

Uncle Toni’s take? “I hope every day we can play a little bit better.”

(Copyright 2016 by The Associated Press. All Rights Reserved.)

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