Jeff Tarango
Former professional tennis player on BBC Radio 5 live sports extra

Nishikori can dig deep. He really pushes himself hard but conserves his energy well. He doesn't show a lot of emotion which saves him energy. He paces himself well and that makes him a good five-set player.

Image source, Getty Images

Kei Nishikori has had a relatively quiet year but is back on top form in New York.

This is his favourite Slam. It is his third semi-final at Flushing Meadows, he hasn't got to this stage at any of the other three majors.

He is also looking to repeat his run to the final here in 2014, his only Slam final. Who did he beat in the semis that year? A certain Mr Novak Djokovic.

Nishikori is also looking to follow compatriot Naomi Osaka in reaching the final this year. There has never been a Japanese man and woman in the final of the same Grand Slam.

Image source, Getty Images

Novak Djokovic is many people's favourite for this year's US Open.

He won Wimbledon in July and dominated Roger Federer to win on the hard courts of Indian Wells last month.

He has dropped just a couple of sets this week but has only played one seeded player, French 26th seed Richard Gasquet.

Today is his biggest test yet.

Right then. The man with the mic is trying to get the crowd back up again for this second semi-final.

Novak Djokovic and Kei Nishikori are introduced to the crowd...

Image source, Getty Images and Reuters

David Law
BBC Radio 5 live sports extra commentator at Flushing Meadows

It is a real shame for the occasion. It is also a shame for Del Potro as it slightly dampens his moment. But if he goes on to win, all will be forgotten.

Russell Fuller
BBC tennis correspondent on 5 live sports extra at Flushing Meadows

Nadal hardly ever retires. Del Potro was playing so well and there was no way back for Nadal. I don't take any pleasure in watching Nadal go through the motions in that second set - especially in case he did more damage. But it was great to see him talking about his future [in the press conference]. Yes, it is bitterly disappointing but he has added another Grand Slam title in 2018 and he is world number one. He knows there is lots to look forward to and he has the patience to keep going. Lets hope he is not out for too long. He might think 'outdoors and the hard courts is too much, I may just save myself for outdoors.' But we will see.

More from Nadal: "I am having two great years so I just have to enjoy the moments. This year has been fantastic until this moment.

"It is close to fighting for titles so that is frustrating. It is tough for me. It is not about losing, it is about not having the chance to fight for it.

"I have fought all my career against these kinds of things too. I have missed a lot of opportunities. It is tough, these moments. But at the same time I have to keep going and working hard to get more opportunities.

"I am playing the right way and enjoying things. I am having success at the age of 32. A lot of people in this room, including me, didn't think I would be at this level at this age. I still have a passion for the game so I will keep fighting and working hard.

"I will not complain. It is like this. I will keep going. That is all."

Image source, Getty Images

One positive is Nadal has been talking about the future in his press conference. There's absolutely no suggestion that this is the end for him.

He sounds understandably despondent however.

Nadal on Del Potro: "I can't say that I am happy because I am not. I will not lie but Juan Martin del Potro is a player who has been through a lot of issues in his career like I have.

"I know how frustrating it is not to be able to do the things you can do. I wish him all the best. It will be huge for him to win again."

Image source, Getty Images

If you are just joining us Rafael Nadal has been forced to retire from his semi-final with Juan Martin del Potro due to a knee injury.

The pair played a brilliant first set before Nadal began to seriously struggle and it was pretty emotional to watch the champion struggle.

The Spaniard eventually retired when Del Potro took the second set.

The second semi-final between Novak Djokovic and Kei Nishikori will begin shortly.

Image source, Getty Images

Nicola: Sad way for it to end after some superb tennis in the match from two greats With all that he's been through though it's so great to see Del Po back fighting at the top of the game and what a story if he could go on to win!

More from Nadal: "I was not feeling worse with my knee before the match or yesterday when I practised.

"The knee is always very similar when it happens. I just have to acceptable it that is all. I cannot compare the knee with other times as the pain is always very similar. This time it was a little bit more aggressive with my movement.

"I don't know what can happen in a couple of days or a couple of weeks. Mentally it is much worse."

Worried it might be structural damage? "No. If I have more things it would be tough. I know what I have. It is a similar thing to always so it is about good treatment. It is not an injury that tells you, 'three weeks off, you are back. Six months off you are back.'

"I know what is going on with the knee and I know what I have to do to be better as soon as possible. We are experienced with that. I am sure it will not be six months off of course, I was just making a comparison."

Rafael Nadal speaking in his press conference: "I had some issues in the tournament. Everyone knows I had an issue in the second or third match.

"It was a little bit better but then yeah. It was in the first set when I felt something on the knee and after that I was just trying to save in some moments to try and improve.

"It was not today. I waited as much as I could and as you can imagine it was very difficult for me to say goodbye before the match finished.

"It was difficult for me to keep playing at the same time. I was in too much pain and it was not a tennis match in the end. It was one player playing and one on the other side of the court.

"I hate retiring but to stay one more set out there playing like that is too much for me."

Image source, Getty Images

Rafa sounds as downbeat as I have ever heard him as he talks about the injury.

He says he "hates" retiring.

Thanks Emma.

Well right now it feels like that time your parents come home and send everyone home from your party. What a disappointing way for that semi-final to end.

Rafael Nadal is speaking right now in his press conference. We'll bring you updates.

The first semi-final may have been cut short but we're still in for a treat.

Up next is Kei Nishikori v Novak Djokovic and Matt Henry is here to take you through it.

Catch you later!

More from Del Potro: "It means a lot. I didn't expect to get to another Grand Slam final. This is my favorite tournament. I had my biggest memories playing on this court but I was a kid. Now I am much older and I will try to enjoy one more day.

On the group of fans that have supported him at the tournament: "Enjoy the final because after the final you have to go home. Thank you everyone and see you on Sunday."

Juan Martin del Potro: "It is not the best way to win a match. I love to play against Rafa because he is the biggest fighter in the sport.

"I don't like to see him suffering like today. I am sad for him. I think the key of the match was the first set."

Image source, Getty Images

Amid the sadness of seeing a great champion trudge off injured, this is a wonderful moment for Juan Martin del Potro.

He has had three wrist surgeries and missed two years of tennis since his last Grand Slam final.

That was in 2009, in New York, which he went on to win.

One champion drops out, another progresses.