Juan Martin del Potro of Argentina hits a return during the men's singles semifinal match against Rafael Nadal of Spain at the 2018 US Open tennis tournament in New York, the United States, Sept. 7, 2018. Juan Martin del Potro won as Nadal retired due to injury. (Xinhua/Liu Jie)

NEW YORK, Sept. 7 (Xinhua) -- The 2009 champion Juan Martin del Potro of Argentina made his second US Open final as defending champion Rafael Nadal retired with two sets down in their semifinal here on Friday.

Del Potro will face either Novak Djokovic of Serbia or Japan star Kei Nishikori in Sunday's men's singles final.

Del Potro took the first two sets 7-6(3), 6-2 when the world number one Nadal retired due to 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."