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.

Image source, Getty Images

A brilliant return from Alcaraz earns him the first point of the tie-break and it's on the Djokovic serve. Early mini-break for the Spaniard.

Djokovic goes for power but his forehand drops wide.

Alcaraz takes the hold as he overpowers Djokovic and the Serb dumps into the net.

We're into a tie-break.

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

I cannot believe how technically perfect Alcaraz's volleys are for someone that young. I just love watching him volley.

To be able to do it so early his career, when some of the top players never felt that comfortable up there.

A lot of information coming from the Djokovic team. He doesn't look too pleased to be hearing it to be honest.

I wonder what advice they have for that 128mph ace Alcaraz has just sent down?

Djokovic does his bit and holds to love. Alcaraz will once again serve to stay in the set.

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

I know we sometimes say that he is playing possum, but I do think Djokovic is looking weary out here. This set is really taking it out of him.

It's his walks in between points for me - the spring isn't quite in the legs like it is with Alcaraz.

Image source, Getty Images

Nerveless from Alcaraz as he holds serve to stay in the set.

Have to say, though, that after just over one and a half hours on Arthur Ashe Djokovic is looking a little tired.

He has fooled us like this before though.

Jonathan Jurejko
BBC Sport senior tennis reporter at Flushing Meadows

Everyone in the 25,000 crowd held their breath as they waited to see how that 27-shot exchange ended.

Djokovic had to catch his afterwards.

But he recovered quickly enough to dig out what could be a huge hold in this fascinating match.

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

If Djokovic wins, he will become the oldest men's Grand Slam finalist since Ken Rosewall over half a century ago in 1974.