Del Potro, Djokovic into U.S. Open final as Nadal retires with knee injury

The 2009 champion Juan Martin del Potro of Argentina made his second U.S. Open final after defending champion Rafael Nadal retired with two sets down in their semifinal here on Friday.

Del Potro will face Novak Djokovic of Serbia, who eased past Japan star Kei Nishikori in straight sets 6-3, 6-4, 6-2 in the other semifinal.

Del Potro took the first two sets 7-6(3), 6-2 when world number one Nadal retired citing knee problems.

While leading 4-3 in the first set, Nadal got his right knee taped but ripped it off later. However, he got it taped again early in the second set when he was obviously underperforming compared to the opening set.

After the Argentine blasted a forehand winner to close out the second set, Nadal could not continue.

"I waited as much as I can. You could imagine it's very difficult for me to say good-bye before the match finishes. But at some point you have to take a decision," Nadal said. "It was so difficult for me to keep playing at the same time that way, having too much pain. That was not a tennis match at the end. I hate to retire, but staying one more set out there playing like this would be too much for me."

"I can't say that I am happy because I am not. I will not lie you. But, yes, he's a player that went through lot of issues during his career, like me," Nadal said of Del Potro. "I know how much frustration can be when you can't do the thing that you want to do. He knows very well. Happy for him that he's able to be back at his top level. For him it will be huge if he's able to win a Grand Slam again."

Del Potro said he felt very sorry for Nadal. He said: "It's not the best way to win a match. I love to play against Rafa because he's the biggest fighter in this sport and I don't like to see him suffering."

"I didn't expect to get to another Grand Slam final in my favorite tournament. I had my biggest memories on this court in 2009 when I beat Nadal in semifinals and Roger Federer in the final. I was a kid then, now I am much older," he added.

In the other semifinal, Nishikori gave all out and was able to make some sizzling shots from baseline, but he still came short in front of the awe-inspiring Djokovic.

Djokovic missed the tournament last year because of an injured right elbow that eventually required surgery. But he is back to his best in time as he bids for his third title at Flushing Meadows and 14th Grand Slam title overall.

Earlier, third-seeded Americans Mike Bryan and Jack Sock defeated No. 7 seeds Lukasz Kubot of Poland and Marcelo Melo of Brazil 6-3, 6-1 in just 74 minutes to win the U.S. Open, giving Bryan his all-time men's doubles record 18th Grand Slam crown.

At 40 years, 4 months, he also passed Indian star Leander Paes as the oldest Grand Slam champion in the men's doubles.