NEW YORK, Sept. 5 -- Top seed Novak Djokovic saved four set points in a marathon first-set tiebreaker to beat Alexandr Dolgopolov of Ukraine 7-6 (14), 6-4, 6-2 to reach the US Open quarterfinals on Monday.

The world No.1 won the tiebreaker 16-14 on his sixth set point, which functioned well on confidence pumping and disheartening.

Dolgopolov wilted soon after the first set defeat and lost the match.

"Winning it was very important," the Serb said. "That was probably the turning point. After that it was a good performance."

"He was changing pace a lot," the Wimbledon and Australian Open champion said. "He was always sending me a different ball. It was really hard for me to adjust to it because of the conditions that we played in.

"That's why it was crucial for me to win the first set. After, I felt a bit more relaxed."

Dolgopolov, who won his first tournament this year and his first matches at a US Open, said he felt good in his first ever match against Djokovic.

"I felt comfortable on the court," he said. "I think I just need to get my first serve, for sure. You can't play a great returner like that, serve around 40 percent, that's impossible.

"I was having fun just stepping on the court, so much people, good court, playing Novak. I made a good result in the tournament. I was just happy to play tennis. I gave a bit of a fight, but I still need some work."

The victory improved Djokovic's 2011 record to a remarkable 61-2 and took him through to a 19th straight grand quarterfinal.

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