Andrew Castle
BBC Sport tennis commentator at the O2 Arena

I don't know how much the players are aware of the stats - it's not vital for Thiem to win the first set of course but Djokovic has not lost a match after winning the first set this year.

Tim Henman
Former British number one on BBC One

Both of these players' serves have improved so much over the years, Djokovic now it's a very simple motion and Thiem likewise. Thiem is not very tall but it's how much power he gets from the leg up.

Nicely done from Domi. Another of this powerful serves finds the mark and Djokovic fails to keep his return under control. Thiem was under a little pressure but holds serve.

This is bubbling along nicely.

We're at deuce now. Thiem found two good serves and a backhand winner but an error prevented him escaping trouble.

A couple of close calls from the technology - there are no line judges this week remember - go against Thiem.

0-30.

The first sign of danger for the Austrian...

With no crowd there's just the noise of racquet on ball and the screech of shoe on court during the point. It must be eerie for the players, no matter how much practice they've got this year.

Djokovic wraps up another routine service game.

Image source, PA Media

Thiem grunts as he whips a forehand into each corner. Too good even for Djokovic.

Carlos Moya, Rafael Nadal's coach, is courtside watching on. A scouting mission - and a sign of confidence - with Nadal playing in the second semi-final later? Or maybe the Spaniard just loves tennis.

Tim Henman
Former British number one on BBC One

Thiem has to find the balance of aggression while keeping the unforced error count low.

Djokovic breezes through his second service game and heads to his chair in front.

There may not be a crowd at the O2 Arena this year but they're putting on quite the light show at the change of ends.

As expected it's Thiem being the more aggressive early on. He's trying to pound away with that big forehand.

He thumps a single-handed backhand too before putting away the volley. Djokovic gives a clap with his racquet in acknowledgment of impressive play.

A good start from both men.

Image source, reuters

Djokovic is dressed all in blue today with his 27-year-old opponent kitted out in black.

Thiem gets an early sighter in the Djokovic's service game, dragging himself to 30-30 before a couple of errors see it go the way of the Serb.

Djokovic begins with a perfect serve - an ace down the middle.

A lengthy point follows with Thiem dictating before Djokovic goes long.

A promising sign of things to come.

And now both men are ready to go.

Predictions time. Thumbs up for Djokovic . Thumbs down for Thiem.

*Denotes next server.

This one is really hard to pick, for me. Thiem was brilliant against Nadal earlier in the week. He's in form.

But is anyone ever the favourite against Djokovic on a hard court? Hmmm.

Tim Henman and Andrew Castle say the Serb is the favourite. Don't forget you can watch this match live on BBC One or by clicking play at the top of this page.

Image source, Reuters

World number one Djokovic is chasing a record-equalling title at the ATP Finals this week.

He has reached a ninth semi-final but there have been signs he has not quite been at his best.

The Serb was beaten by Daniil Medvedev in the group stage and had a couple of iffy moments against Alexander Zverev.

Image source, Getty Images

It has been tennis' unanswered question for what feels like an eternity: when will the Big Three be replaced at the top of the men's game?

Well in the last few months Dominic Thiem has positioned himself as the player most likely.

He broke his Grand Slam duck last month at the US Open and beat Rafael Nadal earlier in this tournament. Next up - Djokovic...

Tim Henman
Former British number one on BBC One

Thiem was really a clay-court player to begin with but he's really transitioned his hard-court game and to have a Grand Slam title under his belt has elevated him. It's going to be a really exciting match-up.

This first semi-final between Novak Djokovic and Dominic Thiem really could be a classic.

They've needed a decider the last three times they have met. Thiem beat Djokovic here last year.

Both men emerge through the dry ice onto into this vast, empty arena.

A year of tennis like no other is drawing to a close and just four men remain in contention for 2020's final prize.

Today, Novak Djokovic, Rafael Nadal, Dominic Thiem and Daniil Medvedev fight it out for a place in final of the season-ending championships.

First up is a mouth-watering clash between the world number one and tennis' newest Grand Slam champion.

Who will end 2020 as king?

Image source, Getty Images