NEW YORK (Reuters) - World No. 1 Novak Djokovic had hoped to have the crowd behind him as he began his bid to win the US Open and complete the calendar-year Grand Slam, but the Serb said the atmosphere was "not ideal" as Flushing Meadows fans roared on his underdog opponent.
Djokovic, looking to become the first man since Rod Laver in 1969 to win all four slams in the same year, beat Holger Rune 6-1, 6-7 (7-5), 6-2, 6-1 to advance to the second round after the 18-year-old Danish qualifier ran himself into the ground against the world No. 1.
Fans chanted "Rune" throughout and applauded the young Dane's bravery as he played on despite being hampered by cramp.
After wrapping up victory Djokovic did not deliver his customary post-match celebration - sharing his heart with all four sides of the court - and he later said he thought the Arthur Ashe Stadium crowd had been booing him.
"I didn't know what they were chanting honestly. I thought they were booing," Djokovic said at a news conference.
"I don't know, it was not ideal atmosphere for me... But I've been in these particular atmospheres before, so I knew how to handle it.
"It's the largest stadium in sport. Definitely the loudest and the most entertaining stadium we have in our sport ... Obviously you always wish to have crowd behind you, but it's not always possible. That's all I can say."
A three-time US Open champion, Djokovic was playing his first match at Flushing Meadows since his disqualification from last year's tournament when he inadvertently hit a line judge with the tennis ball in his fourth round.
A fourth US Open trophy would take him ahead of his great rivals Roger Federer and Rafa Nadal in the Grand Slam count, with the trio all on 20 titles.
"There are things that you can always rely on, go back to when you are experiencing a little bit of a turbulence in terms of emotions on the court or off the court," said Djokovic.
"When it gets too much, then I've developed a mechanism where I feel like I can mentally handle that. I do have a formula for many years that has worked well for me, that got me to where I am. At the same time, it's not necessarily a guarantee that it will work every single time."
German Alexander Zverev got his bid for a maiden Grand Slam title off to a smooth start by beating American Sam Querrey 6-4 7-5 6-2, extending his winning streak to 12 matches.
"I hope I can keep the level up and maybe even play better, because to beat Novak here is going to be an extremely difficult task," Olympic champion Zverev said.
Top seed Ashleigh Barty was made to sweat for a spot in the second round, battling to a 6-1, 7-6 (9-7) win over Russia's Vera Zvonareva.
Belinda Bencic, who won gold in Tokyo last month, made it past the Netherlands' Arantxa Rus 6-4, 6-4 and the 24-year-old Swiss said she had been more relaxed since her Olympic triumph.
"I kind of achieved my dream in Tokyo," Bencic said. "I feel like it gives me a lot of calmness and just relaxing and I can really play just very free now and I'm playing for fun."
Canadian sixth seed Bianca Andreescu prevailed in her first outing at Flushing Meadows since winning the title in 2019. The sixth seed beat Switzerland's Viktorija Golubic 7-5, 4-6, 7-5 in the final match of the day.
Two-time champion Naomi Osaka resumes her title defence by taking on Serbian qualifier Olga Danilovic in the second round on Wednesday, while Russian second seed Daniil Medvedev meets Germany's Dominik Koepfer as he pursues a maiden Grand Slam title.