Novak Djokovic of Serbia celebrates his victory over Alexandr Dolgopolov of Ukraine during their match at the US Open tennis tournament in New York, Sept 5, 2011. [Photo/Agencies]

NEW YORK - 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.

His next opponent is his good friend and Davis Cup teammate Janko Tipsarevic, who beat Juan Carlos Ferrero of Spain in a nearly four-hour struggle.

Tipsarevic powered his way through the match, serving up 20 aces to eight for Ferrero, tipping the scales in what was an otherwise very tightly played match.

Ferrero, a former holder of the world No. 1 ranking, was broken at 5-5 in the third set in what proved to be the turning point in the match.    Tipsarevic won the fourth and final set 6-2 in convincing fashion. Fittingly, Tipsarevic took match point on his 20th ace on the day.

The win sends the 20th-ranked Tipsarevic to the first quarterfinal of his career. He entered the top 20 of the world rankings just last month.

"It means there's going to be at least one Serbian in the semifinals, which is great for our country," Djokovic said.

"I think this is the first time I play Janko in a grand slam. We are great friends. It's not going to be easy to play him, but look, it's the quarterfinals and we both want to win."

Djokovic still has yet to lose a set in the tournament.