Juan Martin del Potro said he could not have imagined he would get another chance to play in a grand slam final, after moving to within one win of a second US Open title.

The champion at Flushing Meadows in 2009, Del Potro reached the final in New York once again when an injured Rafael Nadal, trailing by two sets to love, retired from their semifinal on Friday.

Del Potro would surely have sympathised with Nadal, given the serious wrist problems that threatened to bring a premature end to the Argentinian's career.

He was initially sidelined eight years ago, before missing the bulk of the 2014 and 2015 seasons, unsure whether he would ever return from multiple surgeries.

"The worst moment was in 2015 when I was close to quitting this sport because I couldn't find a way to fix my wrist problems," said Del Potro, who will now face Novak Djokovic or Kei Nishikori in the final.

"Before the last one [operation], I said to myself that could be the last effort because I am not available to keep going to the surgery room again, [and] put my body at risk."

"But I got lucky because it went well, and now it's working well. My wrist, it's okay. I know I'm not 100 per cent, but I can play tennis in these conditions."

"I had been suffering a lot. I got depressed for a couple of months also. I didn't get the chance to feel better with myself, to do this again. That was the bad moment for me."

"But I think that is completely in the past, and now I'm having a good present, looking forward for the future."

"I didn't expect to get these kind of emotions playing tennis again. Reaching finals, winning titles, having my highest ranking ever [third] in this moment. Everything is almost perfect."

As Nadal's struggles became ever more apparent on Arthur Ashe Stadium, Del Potro proved ruthless, pounding winners off both wings.

"I saw him suffering a lot during the second set," he added. "I told myself just to stay focused on my game, don't look at him because you never know what could happen."

"But, of course, when I saw him with bad movements, I started to play aggressive, making him run a lot. Then he decided to stop."