A dejected Rafael Nadal said he had no choice but to retire from his US Open semifinal against Juan Martin del Potro, as his injured right knee was causing him too much pain.

Three days on from his epic five-set victory over Dominic Thiem, the world number one's title defence came to a heartbreaking end as he called a halt to his contest while trailing Del Potro 7-6 [7-3] 6-2.

The opening set went to a tie-break on Arthur Ashe Stadium, but Nadal took a medical timeout early in the second – having already had his troublesome knee taped in the first – and his movement was clearly limited thereafter.

In a news conference on Friday, Nadal said: "I had some issues during the tournament. As everybody knows, I had an issue, it was in the second or third match. Then I think was little bit better."

"I think it was two-all in the first, 15-love, something like this, that I felt [something]. I said to my box immediately that I felt something on the knee. After that, I was just trying to see if in some moment the thing can improve during the match. But no, it was not the day."

"I waited as much as I can. You could imagine [it was] very difficult for me to say goodbye before the match finished. But at some point you have to take a decision. 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, no? It was just one player playing, the other one staying on the other side of the court."

"I hate to retire, but staying one more set out there playing like this will be too much for me."

Asked whether his five-setter with Thiem may have had an impact on his knee, Nadal replied: "You never know. Maybe zero, maybe yes. I was not feeling the worst of my knee today before the match or yesterday when I practiced."

Knee trouble is nothing new for Nadal, who was also forced to retire in the quarter-finals of this year's Australian Open due to a hip problem.

"I'm having two great years. Last year was a fantastic year. This year has been a fantastic year until this moment," he insisted.

"I lost four matches. Two of them I had to retire. The negative thing is two of them have been in the quarter-finals and semi-finals of a grand slam, close to fight for titles. That's frustrating. It is tough for me."

"At the same time, on the other hand, I am going to keep going and keep working hard to keep having opportunities."

"I know things are going the right way. I am playing well. I am enjoying [being] on court. I am having a lot of success. I am very competitive at the age of 32."

"Lots of people in this room, including myself, never will think [sic] that at the age of 32 I will be here fighting for titles, fighting for the first position of the rankings."