Alexander Zverev really is bouncing the ball a lot before he serves. Perhaps it is a tactic to give Karen Khachanov more time to get in his own head.

Whatever it is, it seems to be working as the German sets up two set points.

He only needs one, as Khachanov is again unwilling to advance off the baseline and again eventually shoots long after an extended rally.

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

That was a pretty poor game from Khachanov by his standards.

This could be a pivotal moment in the match. And after the longest rally of the match - 16 shots - Khachanov puts a backhand into the net. Advantage Zverev.

Khachanov gets his first serve in subsequently, but Zverev defends well behind the baseline and converts the first break point of the match as the Russian shoots long.

Image source, Getty Images

Karen Khachanov is trying everything to get around Alex Zverev when he comes to the net, but it is easier said than done. An attempted lob is easily smashed.

At 0-30, Khachanov needs something good - and finds it with a superb serve out wide that Zverev nearly needs to climb into the stands to retrieve.

A crunching forehand cross court winner moves Khachanov to 40-30. That's a big shot and not guaranteed - he's made five unforced errors on his forehand already.

And another mistake comes as he double faults. Deuce.

Image source, Getty Images

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

Zverev is making me nervous bouncing the ball that many times before he serves.

Alexander Zverev has not yet lost a single point where he has advanced to the net. He knows he has more technical skill than Khachanov, and is quicker around the court as long as he plays to his potential and avoids too many unforced errors.

He holds to love.

Image source, Getty Images

Karen Khachanov has only missed his first serve twice in this match so far. Here, he uses his powerful serve to hold to love.

It's hard to see where either man is finding a break of serve at this point.

Image source, Getty Images

Jonathan Jurejko
BBC Sport

New York is known as the city that never sleeps - and so it has been at this year's US Open.

A number of matches have started late and finished in the early hours, backing up the city's reputation as a playground for the nocturnal.

But when some players - including Alexander Zverev - are heading to bed as Manhattan commuters are waking up for work, it raises questions about fairness.

Three of top seed Zverev's five matches over the past fortnight have finished past 1am.

"I've been to bed as late as 6:45am in the past week so I'm kind of a bit jet-lagged," Zverev said. "I feel like I'm playing the Australian Open in New York right now."

This semi-final is the first time the German has played in the day at this year's US Open.

By contrast, unseeded Khachanov has not been on court beyond midnight. He also has slightly longer to prepare after his quarter-final against Alexander Blockx finished shortly before 19:00 local time on Wednesday.

Image source, BBC Sport

Both men have brought their serving A-game so far, both registering five aces between them in the early stages.

Zverev has three, including two in this game as he holds to 15.

Celebrities spotted in the crowd so far include Hollywood actors Woody Harrelson and Matthew McConaughey, plus Brazilian football star Ronaldo.

On court, Karen Khachanov is continuing his hit big and hope strategy. He's up to six unforced errors already.

But when he gets it right, he is forcing Alex Zverev right back - and does so here to hold to 30.

Image source, Getty Images

Alex Zverev is the favourite to win the title, although it has taken time to reach his best level in New York.

The German was taken to five sets in his opening two matches, while he has conceded 80 games at this year's tournament before tonight's match - the second most of any top seed en route to the semi-finals after only Rafael Nadal, who lost 85 games in 2018.

"I think you saw in the first two rounds, I really didn't play well," Zverev said.

"In other tournaments I would be out in the first round, but here I can fight back and play best of five sets, play four-and-a-half hours. I've started playing better throughout the tournament and I'm definitely happy to be in the semi-finals here."

Karen Khachanov knows he won't get anywhere in this match playing on the back foot. He is attempting winners where he can and trying to take the game to Alexander Zverev.

He gets unlucky as a cross-court passing shot clips the net and flies narrowly wide.

Zverev takes advantage of that good fortune, despite a double fault, and holds to 30 - sealed with an ace.

Image source, Getty Images

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

It is interesting to see Khachanov go into that backhand corner as much has he had because Zverev's backhand is just so rock solid and he very rarely misses it.

Zverev looks quite relaxed walking around the court.

Karen Khachanov's second service point of the match sees him send down a 131mph ace - his fastest of the tournament.

But he follows it with the first double fault of the match. A hint of nerves?

He holds to 30.

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

It is interesting to see Zverev trying to play that drop shot and force Khachanov forward.

Karen Khachanov has a chance to move 15-30 ahead with a passing shot, but overcooks it. He winces - he knows opportunities will come at a premium against Alex Zverev.

Zverev holds to 15.

Image source, Getty Images

Alex Zverev, with his man bun high on his head, gets us under way.

If Karen Khachanov wins here, he will be the lowest ranked US Open finalist since ATP rankings were introduced in 1973.

No male player ranked 50 or lower has reached a Grand Slam final since Marcos Baghdatis at the Australian Open in 2006.

Emily Salley
BBC Sport

Unseeded Russian Karen Khachanov advanced to the semi-finals after Alexander Blockx retired trailing 6-2 7-5 3-2 in their last eight match.

It secured his place in a Grand Slam semi-final for the first time since the Australian Open in 2023.

A former world number eight, a disappointing run of results over the past few months have resulted in Khachanov's ranking dipping to 50 and he is unseeded at a major for the first time since 2018.

The 30-year-old said he will have to come up with his "best form" to defeat Zverev and reach his first major final.

"He's been in the top five for so many years. This year he did also step forward. He won his first Grand Slam, played the final in Wimbledon, as well. He's coming up really in hot form," Khachanov said.

Zverev, meanwhile, says he will have to be wary of his opponent and his extensive knowledge of the game.

"We call him [Khachanov] 'The Professor', actually, because he knows everything about tennis, and he knows every single string tension, knows what everybody does, so that's quite fun," Zverev said.

Karen Khachanov wins the toss, and chooses to receive serve first.

Mind games to put a bit of extra pressure on Alexander Zverev?

Image source, Getty Images