Del Potro sets up a cross-court backhand winner but he misses it by a country mile! A wasted challenge confirms that and it's deuce.

Chanda Rubin
American former world number six at Flushing Meadows

That is the type of backhand shot you need the right leg for and Nadal just didn't get it.

A wide backhand slice from Del Potro. Then a forehand winner into the corner.

A missed forehand into the tram lines but a sweet backhand cross-court volley winner follows. 30-30.

Nadal's backhand flies into the net... set point Del Potro.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Delpo serving for the set then. And Rafa has decided on a change of tactic - moving waaaaaay back behind the baseline for his first serves. He's almost stood shoulder to shoulder with the line judges.

Chanda Rubin
American former world number six at Flushing Meadows

Del Potro has cleaned up the unforced errors. He is playing very clean tennis but is still going for his shots.

1/1 break points won by Nadal.

2/3 break points won by Del Potro.

He's done it! Del Potro takes the break and will now serve for the opening set.

What a game!

What a rally!

Del Potro settles into his forehand and Nadal matches him with backhand spin and forehand cross-courts.

The Spaniard tries to mix it up with a drop shot but it isn't good enough and Del Potro punishes it with a pass into the back of the court.

And before you know it, it's two break points for Del Potro.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Nadal didn't show any visible signs of having problems with that troublesome right knee, or at least not that I could see from the media seats in the upper tier.

But he's had a thinnish band of tape wrapped around his leg just below the kneecap.

That's exactly what he had against Karen Khachanov in the third round, although he dismissed it afterwards as a bit of minor soreness.

It didn't hamper him in matches against Basilashvili and Thiem either...

A backhand return from Nadal is immediately swatted away with a one-two punch from Del Potro.

The Argentine then comes into the net for the smash, which bounces over Nadal and into the hands of a very happy man in the stands.

Nadal's backhand goes long - and he loses the challenge - before his banana-whipped forehand around the net goes wide.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Some of Delpo's returns are coming back with a trail of flames. One forehand in that last game drew gasps from the crowd. Unfortunately for the Argentine he was hitting them against a brickwall which was dressed like Rafael Nadal.

Ged Ashton: I hope Del Potro and Nishikori advance to the final instead of having regulars all the time.

The trainer is out...

Nadal needs that knee strapping up again. Is he feeling the heat?

Image source, Getty Images

Chanda Rubin
American former world number six at Flushing Meadows

You have to expect the ball to come back when you play Nadal but it still amazes you.

Oh Del Potro should have won the point!

He smashes an unstoppable forehand into the corner but Nadal (only Nadal) manages to return it into court.

Del Potro isn't expecting it and he can't move his feet in time, volleying beyond the baseline.

Nadal rounds the game off with a backhand winner and holds to love.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Image source, EPA

At times Ashe has a similar atmosphere to a football stadium when Delpo is playing.

"Ole, ole, ole, Delpo, Delpo!" is the favourite terrace chant.

Although the singing of the Argentine fans is not as loud yet as it was during his third-round match against Fernando Verdasco.

That was post-midnight mind. i.e. some of his most vocal fans looked like they had been sinking the free booze in a hospitality box.

Chanda Rubin
American former world number six at Flushing Meadows

That backhand was so sweet. You can't time a ball better than that.

Image source, Getty Images

Del Potro responds with a huge serve down the T which bounces off the rim of Nadal's racquet.

He fires a cross-court forehand and Nadal's backhand is outside the tram lines.

Another deep forehand pushes Nadal so far back he's practically hugging the line judge and the Argentine rounds the game off with a thumping backhand winner.

Del Potro is trying to warm up that forehand. It's fizzing and it's full of depth but it's not quite consistently accurate.

Another overcooked swipe goes wide before he slots one into the net. Nadal leads 30-0...

Monty Moncrieff: Four games in and the first men's semi is already sizzling! Of course I want Rafa to win but I'll be supporting whoever does in the final. Del Potro deserves another Slam too. Vamos! (Both of them.)