By Howard Fendrich
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 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, the top-ranked Djokovic said, "I don't think it's necessary to talk about this now. I'm through. I'm taking it day by day.''
Djokovic hit first serves around 100 miles per hour, sometimes slower — 25 m.p.h. or so below what's normal for him. He hit second serves in the low 80s. 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 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.
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 2½ 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.''
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.''
Puig, seeded 32d, was upset by 61st-ranked Zheng Saisai, 6-4, 6-2. Puig had insisted Saturday that she was mentally ready for the year's final major after the whirlwind of the past couple of weeks. She's now 1-3 at Flushing Meadows.
Two-time US Open runner-up Caroline Wozniacki rallied from a set down for her first Grand Slam match win of 2016. Wozniacki beat qualifier Taylor Townsend, 4-6, 6-3, 6-4, in the first match at the new 8,000-seat Grandstand.
She lost in the first round at this year's Australian Open and Wimbledon and skipped the French Open because of a right ankle injury that forced her to miss 2½ months.
The former No. 1 player's ranking has slipped to 74th, and she's unseeded at Flushing Meadows for the first time since her debut in 2007.
The 20-year-old Townsend, ranked 146th, was playing her second US Open.
The woman who stunned Serena Williams at last year's US Open en route to the final, Roberta Vinci of Italy, won the first match in the refurbished Arthur Ashe Stadium.
Vinci, who is seeded No. 7, overcame some second-set jitters to beat 46th-ranked Anna-Lena Friedsam of Germany, 6-2, 6-4.
In the 2015 semifinals at Flushing Meadows, Vinci — unseeded and ranked 43d at the time — upset Williams, ending the American's bid for tennis's first calendar-year Grand Slam in 27 years. That put Vinci into her first major final, where she lost to another Italian, childhood friend Flavia Pennetta.
In his US Open debut, 21-year-old Brit Kyle Edmund upset 13th-seeded Richard Gasquet.
Edmund, ranked 84th, won, 6-2, 6-2, 6-3, against the 2013 US Open semifinalist. Edmund had 40 winners, 21 on his forehand. He has twice made it to the second round at the French Open.
Gasquet, who hadn't lost in the first round at a major since 2010 at Roland Garros, summed up his quick exit this way: ''Not sure I could do worse.''
Australian Open champ Angelique Kerber advanced to the second round when her opponent retired early in the second set without winning a game.
The second-seeded Kerber led, 6-0, 1-0, after 33 minutes when Polona Hercog stopped. The 120th-ranked Slovenian won just 9 points in seven games.
After the first set, Hercog took a medical timeout. Trainers checked her blood pressure and rubbed ice bags on her legs.
Hercog, who's never been past the third round at a major, came in 2-1 against Kerber in tour-level matches, though they hadn't met since 2011.
Kerber, who lost to Serena Williams in the Wimbledon final, has a chance to overtake her for the No. 1 ranking depending on their results at Flushing Meadows.
Marin Cilic, the 2014 US Open champ, got through to the second round with a straight-set win.
The seventh-seeded Cilic beat 108th-ranked Rogerio Dutra Silva, 6-4, 7-5, 6-1, despite posting more unforced errors than winners (24-23). Cilic is coming off his first Masters title, beating Andy Murray in the final at Cincinnati.