Djokovic is having to work at least twice as hard as Alcaraz to win his points.

An ace will do the trick, though, and he produces one for 30-0.

Woops. Double faults are catching it seems. Alcaraz begins his service game with one but regroups to pummel a forehand winner past Djokovic.

The Serb does not look to be moving particularly well and Alcaraz takes advantage, securing the hold with an ace.

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

Djokovic is weary, and even his camp is weary now. They have stopped shouting their directives at him.

Djokovic saves the first break point but agonisingly concedes the break with another double fault.

That's two in one game and this looks like it's only going one way.

David Law
Commentator on BBC Radio 5 Sports Extra

Alcaraz is trying to do something that no man in the open era has done before - win the US Open without dropping a set.

Rafael Nadal and Ivan Lendl both got to the final without dropping a set, but lost a set in the final.

Four more sets, and Alcaraz has done something never done before.

Image source, getty

Djokovic tries the serve and volley but it doesn't pay off. He's trying things, though.

What he won't have meant to do is skew a forehand off the edge of his racquet on the next point.

That's followed by a double fault and a very tired looking second serve.

Alcaraz has two break points.

A minor scare for Alcaraz at 30-30 but the Spaniard powers ahead and takes the hold with a powerful ace.

Image source, Getty Images

After the excitement of set two it all seems somewhat subdued now. Everyone knows just how difficult this would be for Djokovic to turn around.

He takes the hold to 30 but he knows the body will likely not hold out for a big five setter here.

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

Carlos Alcaraz has almost double the amount of winners, 13 compared to seven.

Looking at the net points, 11 of 18 for Djokovic - this tells you how much he wants to keep the points short. Alcaraz has a very high hit rate with eight out of 10.

Only one more point overall for Alcaraz, isn't that extraordinary?

Is Djokovic going to run out of steam?

Alcaraz takes the hold to 15 in the opening game of this third set.

Djokovic had the trainer out at the end of that set as we thought he might.

But he seems fine to continue and we're under way in set three.

A topsy turvy and at times brutal second set ends with Alcaraz taking it in the tie-break.

What a performance from the Spaniard and if Djokovic didn't have an uphill battle on his hands after set one he certainly does after set two.

Image source, Getty Images

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

Djokovic is still talking to his team even though they are so far away down the other end.

He is gesturing and arguing, he is having a right old rant!

A crucial point for Djokovic to keep himself in the tie-break and the set.

But a ridiculous 131mph serve hands Alcaraz two set points.

Jonathan Jurejko
BBC Sport senior tennis reporter at Flushing Meadows

Oh my word! How did Djokovic reach that drop-shot?

With sheer guts and determination. Then his legs carried him there.

You could hear Djokovic yelp when he realised what was coming his way, before darting forward and leaving Alcaraz scattered on the court.

Image source, Getty Images

What a shot. A sublime pick up from Djokovic at the net and all of a sudden the double mini-break is wiped out and we're back on serve.

A monster rally ends in Djokovic shoving his forehand into the net and that's four points lost on his own serve in the tie-break.

Djokovic floats his return into the base of the net and Alcaraz is firmly in control as he takes the double mini-break.

Djokovic cancels one of those out but it's the Spaniard on top at the change of ends.

Jonathan Jurejko
BBC Sport senior tennis reporter at Flushing Meadows

Djokovic might be losing 16 years to Alcaraz, but he proves he can still chase down short balls like his younger opponent.

Djokovic again beckons the crowd for more noise - they're already giving plenty - as he tries to stay in touch in this decisive tie-break.

What a drop shot from Djokovic to put the tie-break back on serve and he welcomes the noise from the crowd.

Until Alcaraz piles the pressure on and Djokovic's return is into the net. Mini-break restored.

Russell Fuller
Tennis correspondent on BBC Radio 5 Sports Extra and BBC Sounds

The pair's record in tie-breaks this year is strikingly different.

Alcaraz has won 14 and lost eight, Djokovic has only won eight of the 20 he has played. He has lost two of the three in this competition.