Isner wins the first point with a tap at the net but only after he's been made to run around.

Ace.

15-15.

A great serve-and-slap from Del Potro brings up 30-15.

The Argentine then hangs about near the net to sway everything and anything Isner throws at him.

40-15.

Game as Isner goes wide with a forehand return.

Delpo takes the next with a flat double-handed backhand that Isner couldn't stoop to.

Isner has been broken.

Out!

Isner goes long with an angled forehand from the baseline.

Two break points.

He saves one with a big serve that sends Del Potro to Grandstand.

15-0.

Isner then produces a somewhat half-hearted attempt at a double-handed backhand down the line which barely clears two-third of the post.

15-15.

15-30.

The American is struggling.

It's soon 40-0 as Del Potro slaps a forehand winner.

Isner cuts a forlorn figure as he tries to reach for Delpo's serve - he misses as the frame of his racquet smacks the ground.

Game.

Play has resumed.

Delpo wins the first point after Isner's pick-up from the back falls short.

15-0.

Isner is wandering back through the corridors towards court. There are still two more matches to come on Ashe tonight don't forget.

Serena Williams and Karolina Pliskova will be next up at midnight.

Some might disagree - especially, when the fifth doesn't go to a tie-break.

What about if the fifth set of every Grand Slam men's competition is decided by a tie-break?

Both men don't even think about declining the 10-minute break due to the heat rule. They're off court in a flash.

Jeff Tarango
Former professional tennis player on Radio 5 live at Flushing Meadows

John Isner has been playing great tennis this year. His serve, his volley and his intensity are better but he's a little bit older and these long matches will get to him now.

Isner floats a groundstroke back into court. Del Potro winds up. The whole stadium takes an intake of breath. Crunch. That's prehistoric. A thunderous forehand.

Del Potro now roars. He's a set away from another semi-final in New York.

Brilliant from Del Potro! A return we have seen so many times from Rafa and Novak and co, right at the shoe laces of Isner when he serve and volleys.

Three set points to Del Potro. He's got two serves coming up next too.

Boom. A thumping serve from DelPo. He increases his lead.

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Tennis correspondent Russell Fuller and the BBC Radio 5 live tennis team talk all things US Open - in particular whether five-time champion Roger Federer has had his day among the elite of men's tennis.

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Another let-off for Del Potro! He comes into the net but gets the approach all wrong. Isner can't believe he has a chance on the passing shot and makes a mess of it.

The players head for their chairs. 4-2 in the breaker...

There's the break. All of the Americans on Ashe gasp as their man drifts into the tramlines with a forehand.

DelPo edges ahead in this third set tie-break.

Sheeeesh that was quick. A ripper from Del Potro off the forehand wing.

John McEnroe is courtside. His face comes up on the big screen. The crowd love it. I think he likes the applause too.

DelPo gets away with one! Isner comes into the net and should win the point but he gifts DelPo a lifeline.

Could that prove to be a big miss?

Isner to serve first in the tie-break...

Big serve. Big forehand.

Good start from the American.

I'm sure I can hear a few tremors when DelPo winds up and crunches a forehand. It's brutal.

We're going to a tie-break again.