By Julian Linden

NEW YORK (Reuters) - Novak Djokovic clawed his way back from the brink of defeat to beat Roger Federer in an epic semi-final at the U.S. Open on Saturday and join Rafa Nadal in Sunday's championship.

Djokovic, who lost to Federer in New York the past three years, twice came back from a set down and saved two match points to win 5-7 6-1 5-7 6-2 7-5 after a match of unrelenting tension and incredible shotmaking from both players.

"It's one of those matches you'll always remember in your career. I'm so thrilled to be in the final," Djokovic said in courtside interview.

"To be honest, I was just closing my eyes and hitting forehands as fast as I can on the match points ... I managed to come back. I was very lucky."

Djokovic's nail-biting victory robbed the tennis world of what might have been the ultimate grand slam final. Federer and Nadal have played each other in the final at Wimbledon, the French Open and the Australian Open, but no two men have ever met in the finals of all four majors.

"I can only try to imagine the excitement around the two of us maybe playing," Federer said. "I would have loved to play against him here."

For Federer, a five-time winner of the U.S. Open and a finalist last year, it was his earliest exit from New York since 2003, while Nadal moved tantalizingly close to completing his own career slam.

"It's great for tennis and it's great for him at the young age," Federer said. "I won't watch but I hope he wins."

The absence of Federer and the exhausting way Djokovic got through has ensured that Nadal will go into Sunday's final as the overwhelming favorite after he thrashed Russia's Mikhail Youzhny 6-2 6-3 6-4 to reach the final without dropping a set in the entire tournament.

"To me it's a dream. I'm going to play for the first time in the final here in the biggest center court in the world," Nadal said. "I have tried my best for a lot of years. So now after a lot of work I am here and I'm happy for that."

Nadal, armed with a stronger serve and more precise volleys, was in superb form against Youzhny, belting 23 winners and committing just 19 unforced errors.

His only moments of concern were late in the match when he dropped serve for just the second time in the tournament and had his left foot retaped and a bandage placed to ease discomfort from a blister.

Youzhny tried hard but the Russian was also battling fatigue after being pushed to five sets in his previous round.

(Editing by Frank Pingue)