Del Potro is straight back on it. Isner is slow coming into the net and it costs him. Here is another break point to the Argentine...

Now Juan Martin del Potro is back. He's waving his arms around in frustration. He's not happy about something that has gone on in the interval. It was the umpire on the receiving end earlier and now another official is too.

We're going to get back under way after nearly eight minutes without play. Has John Isner had some chocolate to boost his energy levels?

John Isner is the first player to re-emerge. The camera is zooming in on some strapping he has on his left thigh.

Juan Martin del Potro only hit one unforced error in that set. One! Brilliant stuff.

Isner on the other hand hit 15. There's the difference.

We're all square!

Isner doesn't put up much of a fight and goes wide again on Del Potro's first set point.

Both men pick up their bags and head off court. They're going to change their outfits for something a little less sweaty, I assume.

This is a big few minutes for Isner. He needs to find some energy from somewhere.

Well wide from Isner. A shrug of the shoulders. He looks physically spent right now.

Three set points Del Potro...

A ninth ace from Del Potro. The perfect start to the game. 15-0.

Now Isner challenges unsuccessfully on a ball that lands flush on the baseline from the Argentine.

30-0.

Isner is sticking around for now at least in this second set. He forces Del Potro to serve it out...

There's no sign of tiredness from Del Potro. The Argentine, dressed in a bright orange number, pounds down a few more aces and then takes his seat next to an electric fan blowing air into his face. A ball boy quickly scurries over with an ice towel. These players are treated like kings.

The heat rule is in place remember. All of the junior matches were called off earlier because of the heat.

Good response from Isner. He waves his racquet after winning one lengthy point and then finds another ace.

A good hold. Isner is nodding at the back of the court. He has impressed himself there.

Isner's service speeds were closing on 140mph earlier but are now about 15mph slower as he continues to look drained. An effort at 117mph is right in DelPo's arc and he gobbles up a return winner. Now a double fault from the American...

Deuce.

Isner is creaking and lumbering around the back of the court like one of those walking trees in Lord of the Rings. If this is an issue due to the heat then it has hit Isner very quickly. All of a sudden his game has dropped away. Del Potro comfortably increases his advantage.

That one is out though! A clench of the fist from Del Potro. Isner bends over double behind the baseline and then waves a hand in frustration.

These are worrying signs for the American.

One break point saved.

15-40.

Juan Martin del Potro is a little sweaty but John Isner's green, white and blue striped shirt is absolutely soaked with sweat.

He's looking tired all of a sudden. Here are three break points for Del Potro. The American can't find a first serve...

It seems this issue is Del Potro wants to change his shorts. He's not allowed to do that on court and hasn't been allowed to nip off at the change of ends.

It can't be easy being a line judge in this match. The ball is flying off the racquet of both men on serve and thudding into the backboard behind the receiver. Not only is it tough to make the right call, you wouldn't blame the officials for fearing for their safety a little.

We've already had two challenges in the opening stages of this second set and remarkably both have been right. Impressive stuff.

An easy hold for DelPo. Umpire Carlos Ramos is getting another ear full though.

Oh. How entirely predictable. Two booming aces and Isner saves the break point and holds.

Hello. This could lift Del Potro's mood. A break point...

DelPo had an angry word with the umpire at the end of that last set. The conversation was in Spanish so I can't bring you any details sadly.

He bashes down a few serves to get on the board in this set and grumpily wanders for a drink.