Jonathan Jurejko
BBC Sport at Flushing Meadows

Image source, BBC Sport

With a population of 8.5m, New York is a city which needs to be kept moving. And with the roof on Arthur Ashe Stadium open, it is easy to hear above the sound of tennis balls pounding strings.

The silence during points is often punctured by noisy planes just taking off from nearby La Guardia airport and express trains on the Long Island Railway heading into Manhattan.

I had read that planes were redirected around the Billie Jean King National Tennis Center during the US Open - not today.

Do you like rallies? Because we are getting A LOT of rallies.

Del Potro loses out this time as his forehand crashes into the net and Nadal backs it up with a smashing serve down the T.

The Argentine keeps pushing with his forehand and Nadal eventually miscues into the net. But the Spaniard serves up a treat to set up game point.

He wraps it up with a soft backhand volley and gestures for his bath towel.

Del Potro rolls his eyes as a forehand hits the top of the net cord and bounces back on his side of the court. 30-15 to Nadal.

But a thumping serve draws Del Potro level. He hits a big forehand cross-court and it forces Nadal to overhit his return.

A stern look from the Spaniard and a shake of the head...

Del Potro then produces one-two punch and takes the game.

Chanda Rubin
American former world number six at Flushing Meadows

This first set is much more important to Del Potro.

Nick: Nadal is standing so far back to receive serve he should be charged admission.

Nadal fires himself ahead to 30-0 but Del Potro plays a deep forehand which lands on the edge of the baseline.

Under pressure, Nadal fires a forehand into the corner and Del Potro's top-spin return is long. Game Nadal.

Del Potro fires an excellent forehand return towards the baseline. It's called out but immediately overruled.

Nobody really knows what's going on so the point is replayed. The Argentine is frustrated.

Chanda Rubin
American former world number six at Flushing Meadows

There are not many players who can handle Del Potro's serve like Nadal just did.

Jonathan is right.

Nadal snatches his break point at the first time of asking and it's all square again.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Not the start that Nadal was hoping for. The positive? It was better than his opening game against Dominic Thiem in his quarter-final.

On that evidence he isn't going to be bagelled in 25 minutes.

Excellent defence from Nadal.

He goes 30-0 up with a forehand passing shot after Del Potro had moved towards the net and was set-up for the volley.

The third seed bounces back with a fizzing forehand into the corner but he scuffs his double-handed backhand into the net.

Two break back points for Nadal.

Chanda Rubin
American former world number six at Flushing Meadows

What a big game. Del Potro is ready to take advantage of the slightest opportunity.

One break point saved...

But Nadal's movement is off. He can't settle and Del Potro forces the Spaniard to run from side-to-side before his forehand smashes into the net.

Break for Del Potro.

Jonathan Jurejko
BBC Sport at Flushing Meadows

It's like a boxing match before the bell goes here. Loud rock music. Flashing spotlights. All that is missing is Michael Buffer. Let's get ready to rumble.

Nadal's first two forehands are smacked into the net but his third is a sublime winner down the line.

Del Potro is equal to it - opening his shoulders and playing some smart cross-court shots. Two break points for the Argentine.

We've already had some excellent rallies, some fizzing forehands and a thumping Nadal fist pump.

It's going to be a looooooong night.

Cat Archer
BBC Radio 5 live sports extra on Arthur Ashe Stadium

Gigi Salmon, Chanda Rubin and David Law your commentary team for Del Potro against Nadal.

No popcorn in the media canteen...so I’ve had to give them nuts instead.

Image source, BBC Sport

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

This feels like a fight for the heavyweight championship of the world.

The first men's semi-final of 2018 is underway.

Rafael Nadal to serve first.

*Denotes the server.

We're going to be treated to some fantastic tennis tonight.

But who do you think will book their place in Sunday's final? I want to know your thoughts on tonight's matches and the US Open tournament in general.

Send your thoughts in using #bbctennis

Jonathan Jurejko
BBC Sport at Flushing Meadows

These two players can just not avoid each other at the Slams recently. And it has come at Del Potro's cost.

This is their fourth meeting in the past five Slams - and Delpo has lost all three so far:

• 2017 US Open - Nadal won 4-6 6-0 6-3 6-2
• 2018 Roland Garros - Nadal won 6-4 6-1 6-2
• 2018 Wimbledon - Nadal won 7-5 6-7 (6-8) 4-6 6-4 6-4
• 2018 US Open - ??????