Novak Djokovic opened the defence of his US Open title with a 6-3, 5-7, 6-2, 6-1 win over Poland’s Jerzy Janowicz on Monday, but the laboured performance gave rise to fresh concerns about the world number one’s fitness.
 After a sizzling start to the season that brought grand slam wins No. 11 and 12 at the Australian and French Opens, Djokovic’s form has plummeted, with a third-round loss to Sam Querrey at Wimbledon followed by a first-round exit at the Rio Olympics.
 The Serb might have been in trouble on another day at Flushing Meadows but Janowicz, ranked 246 places below Djokovic, has advanced from the first round just once in four previous US Open visits and looked unlikely to do it again.
 Djokovic, a US Open finalist five of the last six years, next faces Czech Jiri Vesely, a five-set winner over Indian qualifier Saketh Myneni.
 Djokovic arrived in New York having been hampered by a left wrist injury and distracted by undisclosed “private matters” and on Monday trainers were called out early in the opening set to work on his right forearm.
 Several times during the two-hour, 37-minute match, Djokovic could be seen grimacing when hitting his powerful forehand, while his serve rarely looked threatening, stuck at around 100mph.
 “It was just prevention, it’s all good,” Djokovic told reporters. “Look, each day presents us some kind of challenges that we need to accept and overcome. After all I’ve been through in the last couple of weeks it’s pleasing to finish the match and win it.”
 Up 3-2 in the opening set, Djokovic called for a medical time out during the changeover as Janowicz took a seat in the stands while the world number one had his forearm massaged. When play resumed, Djokovic immediately broke the Pole and held serve on way to taking the first set.
 But in the second frame Djokovic’s discomfort became even more evident as he double-faulted and then sent a wild forehand long as Janowicz broke to take control and level the match.
 Normal service was resumed in the third and Djokovic moved in for the knockout punch, breaking Janowicz to open the fourth before wrapping up the match.
 Earlier, the year’s final grand slam got off to a glitzy Hollywood-style start, with a performance from Phil Collins to mark the arrival of the $150-million retractable roof at the stadium.
 “It’s hard to put on show after Phil Collins,” said Djokovic, before breaking into one of Collins’ hits during his on court post-match interview.
“The US Open is the most entertaining grand slam. It was wonderful to come back and play a night session that is undoubtedly the most special night session we have in our sport.”
 Elsewhere, the Serb’s long-time rival Rafael Nadal shone in 33-degree heat while easing to a 6-1, 6-4, 6-2 win over Denis Istomin of Uzbekistan. Nadal, the 2010 and 2013 champion here, is seeded to face Djokovic in the semi-finals.
 Djokovic next faces Jiri Vesely, the Czech who knocked him out of Monte Carlo this year. Nadal’s reward for a fifth win in five meetings with Istomin is a match-up with Italian veteran Andreas Seppi.
 A wrist injury forced the Spaniard into an early retirement at the French Open and sidelined him from Wimbledon before a gold medal run in the doubles at the Rio Olympics helped ease his misery.
 In the women’s event, second-seeded Angelique Kerber, the Australian Open champion, needed just 33 minutes to progress but Roland Garros winner Garbine Muguruza required three sets and a visit from the doctor before she survived.
 Kerber needed just over 30 minutes to get her pursuit of a first US Open title off to a winning start.
The German was leading Polona Hercog 6-0, 1-0 when the Slovenian called it quits having won just nine points. Kerber, a semi-finalist in 2011, next faces Mirjana Lucic-Baroni of Croatia.
 Muguruza carved out a gritty 2-6, 6-0, 6-3 win over Belgian qualifier Elise Mertens, ranked 137th.
But the French Open champion needed courtside treatment from a doctor after dropping the first set. The consultation certainly worked as she reeled off the next seven games to lay the foundation for her win. Muguruza next plays Anastasija Sevastova of Latvia for a place in the last 32.
Yesterday, women’s 29th seed Ana Ivanovic of Serbia lost 7-6(4) 6-1 to Denisa Allertova of Czech Republic in the first round to cap a misarable season, while fifth seed Simona Halep of Romania looked ominous, losing just two games en route to a 6-0, 6-2 win over Belgium’s Kirsten Flipkens.
 French 13th seed Richard Gasquet, a semi-finalist three years ago, was the opening day’s biggest casualty. He went down to 6-2, 6-2, 6-3 to British world number 84 Kyle Edmund.
 Also exiting was Olympic champion Monica Puig with the Puerto Rican losing 6-4, 6-2 to China’s Zheng Saisai. Kayla Day, just 16 and ranked at a lowly 374 in the world, was 6-2, 4-2 up on Madison Brengle when her fellow American quit with an injury.
 Day next faces American eighth seed Madison Keys who beat compatriot Alison Riske 4-6, 7-6 (7/5), 6-2 in a tie which ended at 1.48am, the latest finish to a women’s match in tournament history.
 Croatian seventh seed Marin Cilic, the 2014 champion and winner in Cincinnati last week, eased past Brazil’s Rogerio Dutra Silva 6-4, 7-5, 6-1.
 Flamboyant French 10th seed Gael Monfils took his summer hardcourt record to 15-2 with a 6-4, 6-2, 7-6 win over Gilles Muller of Luxemburg. Monfils almost suffered a freak accident when a giant courtside clock fell towards him as he collided with the back wall, chasing down a Muller shot.

Results (Round I)
Men:
Novak Djokovic (SRB x1) bt Jerzy Janowicz (POL) 6-3, 5-7, 6-2, 6-1; Jiri Vesely (CZE) bt Saketh Myneni (IND) 7-6 (7/5), 4-6, 2-6, 6-2, 7-5; John Isner (USA) bt Francis Tiafoe (USA) 3-6, 4-6, 7-6 (7/5), 6-2, 7-6 (7/3); Steve Darcis (BEL) bt Jordan Thompson (AUS) 5-7, 3-6, 7-6 (7/5), 7-5, 7-5; Ernesto Escobedo (USA) bt Lukas Lacko (SVK) 6-4, 4-6, 4-6, 6-3, retired; James Duckworth (AUS) bt Robin Haase (NED) 4-6, 7-5, 6-3, 4-6, 6-3; Vasek Pospisil (CAN) bt Jozef Kovalík (SVK) 6-1, 6-3, 6-3; Jo-Wilfried Tsonga (FRA x9) bt Guido Andreozzi (ARG) 6-3, 6-4, 6-4; Kevin Anderson (RSA x23) bt Yoshihito Nishioka (JPN) 6-3, 7-5, 6-4; Jack Sock (USA x26) bt Taylor Fritz (USA) 7-6 (7/3), 7-5, 3-6, 1-6, 6-4; Mischa Zverev (GER) bt Pierre-Hugues Herbert (FRA) 6-4, 7-6 (8/6), 4-6, 6-0; Sergiy Stakhovsky (UKR) bt Gastao Elias (POR) 6-1, 3-6, 2-6, 6-3, 7-6 (7/4); Marin Cilic (CRO x7) bt Rogério Dutra Silva (BRA) 6-4, 7-5, 6-1; Rafael Nadal (ESP x4) bt Denis Istomin (UZB) 6-1, 6-4, 6-2; Andreas Seppi (ITA) bt Stéphane Robert (FRA) 6-2, 3-6, 6-4, 6-3; Andrey Kuznetsov (RUS) bt Thomaz Belluci (BRA) 6-4, 3-6, 6-1, 7-6 (8/6); Albert Ramos (ESP x31) bt Julien Benneteau (FRA) 3-6, 6-3, 6-3, 2-6, 6-1; Lucas Pouille (FRA x24) bt Mikhail Kukushkin (KAZ) 3-6, 6-2, 6-4, 6-2; Roberto Bautista Agut (ESP x15) bt Guillermo García-López (ESP) 7-5, 6-1, 6-7 (5/7), 7-6 (7/3); Gael Monfils (FRA x10) bt Gilles Muller (LUX) 6-4, 6-2, 7-6 (7/5); Pablo Cuevas (URU x18) bt Dudi Sela (ISR) 6-3, 6-2, 0-6, 5-7, 6-3; Benoît Paire (FRA x32) bt Dusan Lajovic (SRB) 6-2, 2-6, 3-6, 7-5, 6-1; Marcos Baghdatis (CYP) bt Facundo Bagnis (ARG) 6-4, 6-2, 1-1 retired; Milos Raonic (CAN x5) bt Dustin Brown (GER) 7-5, 6-3, 6-4
Women: Kayla Day (USA) bt Madison Brengle (USA) 6-2, 4-2 retired; Duan Ying-Ying (CHN) bt Maria Sakkari (GRE) 6-4, 4-6, 6-3; Monica Niculescu (ROM) bt Barbora Záhlavová Strýcová (CZE x18) 6-3, 6-7 (3/7), 6-1; Svetlana Kuznetsova (RUS x9) bt Francesca Schiavone (ITA) 6-1, 6-2; Johanna Konta (GBR x13) bt Bethanie Mattek-Sands (USA) 6-3, 6-3; Tsvetana Pironkova (BUL) bt Virginie Razzano (FRA) 6-1, 6-2; Andrea Petkovic (GER) bt Kristina Kucova (SVK) 7-6 (7/3), 6-3; Zheng Saisai (CHN) bt Mónica Puig (PUR) 6-4, 6-2; Garbine Muguruza (ESP x3) bt Elise Mertens (BEL) 2-6, 6-0, 6-3; Roberta Vinci (ITA x7) bt Anna-Lena Friedsam (GER) 6-2, 6-4; Yulia Putintseva (KAZ) bt Sabine Lisicki (GER) 6-1, 6-2; Carina Witthoeft (GER) bt Misaki Doi (JPN x30) 6-4, 6-1; Lucie Safarova (Czech) beat Daria Gavrilova (Australia) 6-4, 6-4; Denisa Allertova (Czech) beat 29-Ana Ivanovic (Serbia) 7-6(4), 6-1; 5-Simona Halep (Romania) beat Kirsten Flipkens (Belgium) 6-0, 6-2
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