Zverev is given a warning for taking too long to serve. Talk about timing.

What follows is unquestionably point of the match - a high quality rally, Zverev seems certain to have won it with a crunching forehand... only for Khachanov to dig out the kind of backhand winner down the line tennis players dream of.

Zverev wins the next point with a volley. He has a third set point.

Forehand blown almost to Canada by Khachanov. His 15th unforced error on his forehand.

Zverev will serve for the set.

Annabel Croft
Former British number one on BBC Radio 5 Sports Extra 2

Khachanov did so well there.That was a blistering shot.

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Brilliant forehand from Khachanov when the pressure was on. 6-6.

Now this feels like a Grand Slam semi-final!

135mph ace from Zverev ties the tie-break. 5-5.

And a 116mph second serve to follow is a risk rewarded for the German, 6-5. Set point.

Could this be a huge moment? Khachanov, 3-1 down in the breaker, lands a massive forehand right on the baseline and suddenly leads at this crucial juncture.

Zverev with a backhand wide, 3-5. It came in another lengthy rally - in rallies of nine shots or more, the Russian has won the majority.

But a tight backhand into the net from Khachanov means we are back on serve.

Alexander Zverev has won 28 tie breaks this year. Khachanov has only won eight.

Zverev has absolutely found his range on serve, another ace - his sixth of the set. The top seed has found his best at the right time, punching the ball into the corners and leaving Khachanov with no chance.

But the Russian is not lying down, and some big hitting makes sure they are level at the change of ends.

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A fourth ace of the second set, and seventh of the match, moves Zverev 30-0 ahead in this important game.

Then another to see it out!

He's found some of the flow last seen in set one at the right time, holding to love. Tie break time.

Annabel Croft
Former British number one on BBC Radio 5 Sports Extra 2

Khachanov looks more free, the way he is playing and moving and you can sense that internally, he is more comfortable with the scenario he is in.

A screaming winner down the line from Khachanov. While Zverev is struggling for every point, especially on his serve, the Russian's game is flowing far better at this point.

He holds to 15.

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Annabel Croft
Former British number one on BBC Radio 5 Sports Extra 2

It is quite laboured, this service game.

Alexander Zverev hits his fourth double fault of the match, and third of the set. He has looked tight at times today.

Then Zverev dumps a forehand into the net from close range. 15-30, as the German's previously flawless record at the net has eluded him entirely.

Zverev fights back to 30-all, then is rewarded for a 120mph second serve out wide. That's a key point won.

He sees it out, and on we go.

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For the first time in the match, the players are being distracted by movement in the stands. The crowd had been pretty well behaved until that point.

Khachanov can't let it distract him, and he doesn't as he sends in his first ace of the second set on his way to a hold to 15.

Zverev must serve to stay in the set.

Alexander Zverev is not a happy bunny, and needs to channel that into his game. A searing cross-court forehand beats Khachanov all ends up.

But on the following point, Zverev hits the deck! It's just a slip and loss of footing, he gives a shy grin and seems to have only hurt his pride.

And he holds to 15, adjusting well at the net after a heavy cord before an overhead smash winner.

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I'm not sure Karen Khachanov has hit a single drop shot in this match. In that 32-shot rally, every shot was towards the baseline - he knows if he can push Zverev back with his power, that is his best chance of victory. Attack is the best form of defence it seems.

He holds to love. Pressure back on the German.

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Bang. 140mph serve from Zverev. Advantage.

And he seals the game with a lovely, leaping overhead backhand at the net. Well played.

Annabel Croft
Former British number one on BBC Radio 5 Sports Extra 2

That has got to be one of the best exchanges we have seen in the entire match. A really physical rally and mentally applying themselves to each other. That was a very significant point. Maybe that will adjust what is going on in this match.

Alexander Zverev was allowed to dictate the court in set one, particularly in his service games. Karen Khachanov has taken a step forward from the baseline to stop that, and it is now up to the top seed to react.

Hmm, a 117mph second serve into the net will not fit that bill. 15-30.

Khachanov however cannot press his advantage with the court gaping in the next point. The Russian is getting chances, and is not taking all of them.

But how about now? A long rally, 32 shots, Khachanov seizes control with big hitting and advances forward at the right time for a forehand winner. Break point.

Zverev with an unreturnable serve denies it. Deuce.

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The US Open is this year offering the biggest prize fund in Grand Slam history with a total of $108m (£79.2m) up for grabs - marking a 20% increase on last year.

The singles' champions will receive $5.5m (£4.03m) each in a 10% increase on 2025.