New York - Defending champion Novak Djokovic overcame an arm injury scare and long-time rival Rafael Nadal shone in 33-degree heat as the US Open showcased its $650 million upgrade on Monday.
World number one Djokovic, also the 2011 winner in New York and chasing his third major of the year, defeated Jerzy Janowicz of Poland 6-3, 5-7, 6-2, 6-1.
Nadal, the 2010 and 2013 champion and seeded to face Djokovic in the semi-finals, eased to a 6-1, 6-4, 6-2 win over Denis Istomin of Uzbekistan.
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.
Djokovic had admitted that he was "not 100 percent" after suffering a wrist injury on the eve of the Olympics where he was a first-round loser.
But it was the upper right arm that bothered the Serb on Monday night as he played his first Grand Slam match since his shock third round exit at Wimbledon.
The 29-year-old needed treatment after just five games of the first set but eventually pulled through against Janowicz who had played just two matches all year due to a knee injury.
"It was hard to follow Phil Collins," joked Djokovic in reference to the lengthy opening ceremony which featured the veteran English rocker under the new $150 million roof on Arthur Ashe Stadium.
On his arm injury, Djokovic added: "It's never easy to play at this level throughout the year. There are periods where you aren't 100 percent."
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 Olympics helped ease his misery.
"The most important thing is that I am here in New York. Injuries are part of the career. I had a hard time this year," said the fourth seed.
Canadian fifth seed Milos Raonic, the runner-up to Andy Murray at Wimbledon, overcame German dangerman Dustin Brown 7-5, 6-4, 6-4.
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 at 137.
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 plays Anastasija Sevastova of Latvia for a place in the last 32.
French 13th seed Richard Gasquet, a semi-finalist three years ago, was the 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.
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.
The Flushing Meadows complex has been radically and expensively remodelled for this year.
However, the old Grandstand court, due to be demolished after the tournament, was pressed into action after the new Court 10 was deemed unplayable.
Results from the first day of the 2016 US Open tennis championships at Flushing Meadows on Monday (x denotes seeded player):
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
Guido Pella (ARG) bt Bjorn Fratangelo (USA) 6-3, 6-4, 6-4
Mikhail Youzhny (RUS) bt Martin Klizan (SVK x28) 6-2, 6-1, 6-1
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
Kyle Edmund (GBR) bt Richard Gasquet (FRA x13) 6-2, 6-2, 6-3
James Duckworth (AUS) bt Robin Haase (NED) 4-6, 7-5, 6-3, 4-6, 6-3
Vasek Pospisil (CAN) bt Jozef Kovalk (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 Rogrio 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 Stephane 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
Marco Chiudinelli (SUI) bt Guilherme Clezar (BRA) 2-6, 7-6 (8/6), 6-2, 6-4
Federico Del Bonis (ARG) bt Brian Baker (USA) 3-6, 6-7 (6/8), 6-4, 6-2, 6-2
Roberto Bautista Agut (ESP x15) bt Guillermo Garca-Lopez (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)
Jan Satral (CZE) bt Mackenzie McDonald (USA) 4-6, 3-6, 6-3, 6-2, 6-2
Nicolas Almagro (ESP) bt Marton Fucsovics (HUN) 6-1, 6-4, 7-6 (9/7)
Pablo Cuevas (URU x18) bt Dudi Sela (ISR) 6-3, 6-2, 0-6, 5-7, 6-3
Benoit 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
Ryan Harrison (USA) bt Adrian Mannarino (FRA) 6-4, 7-6 (7/5), 6-3
Milos Raonic (CAN x5) bt Dustin Brown (GER) 7-5, 6-3, 6-4
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
Naomi Osaka (JPN) bt CoCo Vandeweghe (USA) 6-7 (4/7), 6-3, 6-4
Monica Niculescu (ROM) bt Barbora Zahlavova Strycova (CZE x18) 6-3, 6-7 (3/7), 6-1
Ana Bogdan (ROM) bt Sorana Cirstea (ROM) 0-6, 7-5, 6-2
Caroline Wozniacki (DEN) bt Taylor Townsend (USA) 4-6, 6-3, 6-4
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
Belinda Bencic (SUI x24) bt Samantha Crawford (USA) 6-7 (6/8), 6-3, 6-4
Zheng Saisai (CHN) bt Monica Puig (PUR) 6-4, 6-2
Kateryna Bondarenko (UKR) bt Hsieh Su-Wei (TPE) 6-0, 7-5
Anastasija Sevastova (LAT) bt Anna Karolna Schmiedlov (SVK) 6-3, 6-3
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
Christina McHale (USA) bt Mona Barthel (GER) 6-2, 6-2
Yulia Putintseva (KAZ) bt Sabine Lisicki (GER) 6-1, 6-2
Carina Witthoeft (GER) bt Misaki Doi (JPN x30) 6-4, 6-1
Lesia Tsurenko (UKR) bt Irina Begu (ROM x21) 6-0, 6-4
Wang Yafan (CHN) bt Alison Van Uytvanck (BEL) 6-3, 7-6 (7/4)
Evgeniya Rodina (RUS) bt Danielle Collins (USA) 6-1, 6-2
Dominika Cibulkova (SVK x12) bt Magda Linette (POL) 6-2, 6-3
Petra Kvitova (CZE x14) bt Jelena Ostapenko (LAT) 7-5, 6-3
Cagla Buyukakcay (TUR) bt Irina Falconi (USA) 6-2, 6-1
Lauren Davis (USA) bt Aliaksandra Sasnovich (BLR) 6-2, 4-6, 7-5
Elina Svitolina (UKR x22) bt Mandy Minella (LUX) 6-7 (5/7), 6-0, 6-2
Shelby Rogers (USA) bt Sara Errani (ITA x27) 6-4, 7-6 (7/3)
Catherine Bellis (USA) bt Viktorija Golubic (SUI) 6-2, 6-3
Mirjana Lucic-Baroni (CRO) bt Alize Cornet (FRA) 6-4, 6-1
Angelique Kerber (GER x2) bt Polona Hercog (SLO) 6-0, 1-0 retired