Novak Djokovic was playing against more than just Danish qualifier Holger Vitus Nodskov Rune to advance past the first round of the U.S. Open on Tuesday night.

Djokovic is attempting to win the calendar-year Grand Slam with a win at the U.S Open but on Tuesday he was challenged by the crowd when he interpreted chants for 18-year-old Rune as boos. 

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"I didn’t know what they were chanting honestly. I thought they were booing," he told reporters after the match, via FOX Sports Australia. "I don’t know, yeah, it was not ideal atmosphere for me to tell you that. But I’ve been in these particular atmospheres before, so I knew how to handle it."

Even if they weren’t booing, fans were certainly cheering against him. 

"I mean, obviously you always wish to have the crowd behind you, but it’s not always possible," he said. "That’s all I can say. I mean, I don’t know; I’ve been focusing on myself and what I need to do."

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The chants were a surprise even to Rune, who like Djokovic, thought the crowd was booing.

"At one point they were cheering for me; another point they were booing," he said. "I was like, ‘What is going on?’ All of a sudden, after the match I heard they said, ‘Rune.’ When I heard that, I was happy because I didn’t understand it in the match."

Djokovic dropped a set on Tuesday but advanced to the second round after defeating Rune 6-1, 6-7 (5), 6-2, 6-1.

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"It wasn’t the best of my performances," he said before adding, "I guess I have to just see how it feels on the court and try to keep it together. That’s all I can do."

The Associated Press contributed to thIs report.