Serbian third seed Novak Djokovic [Images] survived a tough challenge from big-serving Croatian teen-ager Marin Cilic 6-7, 7-5, 6-4, 7-6 in a late-night slugfest to reach the fourth round of the U.S. Open on Sunday.

Djokovic, the 21-year-old Australian Open [Images] champion who lost last year's Open final to Roger Federer [Images], was matched stroke for stroke by the 19-year-old Cilic, seeded 30th and playing in his first U.S. Open.

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They battled fiercely in the first set in long rallies that drew gasps from the crowd, with Cilic showing his youthful speed and extraordinary reach to retrieve what seemed like certain winners.

Both players saved set points in the opening tiebreaker before Cilic used a big serve and powerful groundstrokes to win it 9-7.

The next two sets were hard fought but 21-year-old Djokovic was more consistent, forcing Cilic to run from corner to corner, wearing him down.

The world number three served for the match at 5-4 and had two match points from 40-15, but Cilic came through with great returns of first serves followed by sizzling groundstrokes to save them and even the set.

In the decisive tiebreaker, Djokovic delivered a knockout blow, steam-rolling through to win it 7-0.

"I had a very, very difficult match," Djokovic said courtside. "I used my chances, I stayed focused. I'm really happy."

Next up for Djokovic will be 15th seed Tommy Robredo of Spain.

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