Djokovic may be unbeaten indoors since 2012, he may be on a seemingly never-ending winning run - but how many titles has he won indoors, eh? Not as many as Roger Federer, who has 22 well-polished prizes in the bag. 

In fact, no-one has a better win-loss percentage record indoors than the Swiss who has won 261 of his 324 matches under the protection of a roof. 

Federer serving with a surgeon's precision, hitting his targets, racing to 40-0. There's a blip - which Djokovic eyeballs and returns with gusto, then a double fault. 40-30. But the third seed pulls through. No fireworks yet. 

Image source, BBC Sport

Ros Clarke: Can't see anyone beating Djokovic this week, tbh. Unless it's Wawrinka in another one of those flukes...

Andy Jamieson: Djokovic (sadly). Fed is sensational but not quite as much as Djokovic.

Boris Becker crouched forward in his seat. Djokovic's coach is wearing a fetching scarlet tracksuit top this evening. 

Two easy opening points from Djokovic. There's a hint that Federer will make a contest of it when he nicks a point for 30-15, but the Swiss scoops a backhand into the crowd, giving Djokovic a much-needed breather. 

But - whoa! Incredible from Federer. An unstoppable return down the line. But the Serb serves out for a routine hold. 

Max Baggins-Craig: This will be without doubt a brilliant match, fantastic few days we're having, tennis never lets you down.

Miles Maclagan
British tennis coach on Radio 5 live sports extra

A rousing chorus of approval greets Federer as he launches an ace to the wings to secure the hold. Djokovic didn't have a sniff from deuce onwards. 

Federer is the man testing the bounce of the yellow balls, which suggests he will be the man getting this match under way. 

The Swiss aggressive from the get-go, prowling to the tape on the second point - only he miscues his volley to allow Djokovic to level at 15-15. Federer to the net again on the next point, manipulating his opponent before seeing him off with a smasher of a smash. 30-15. 

Djokovic, though, hanging in, impressive retrieving drags him to 30-30 and a botched Federer forehand earns him a break point. The world number three, though, serves and volleys his way out of danger. To deuce? Let's.  

Fancy a chat? Fancy 'getting involved'? Excellent. This evening's question is simple enough - who do you think will win this skirmish and why? Tweet your thoughts using the hashtag #bbctennis. I'll be nice to hear from you. 

I know these stats that are being chucked at you might seem Novak Djokovic-centric, but that's because they are, because they're out of this world. 

The world number one has won 10 titles this year - including three of the four Grand Slams - and has reached a record 14 consecutive finals in a season.

In terms of the ATP World Tour, the top seed has won 15 straight matches, which includes victories over Federer in 2012 and 2013 (but not including the Swiss' walkover in the 2014 final.)

Image source, Getty Images

As we've mentioned, being the best in the business means Novak Djokovic and Roger Federer often go eyeball to eyeball on the tennis court. 

When it matters most, when the stakes are high, the pressure intense, Djokovic often pulls through, beating the Swiss en route to six of his 10 Grand Slam titles. 

Image source, BBC Sport

Miles Maclagan
British tennis coach on Radio 5 live sports extra

Piers Newbery
BBC Sport at the O2 Arena in London

If Novak Djokovic has discovered the answer to life, the universe and how to relentlessly hit the baseline, he's not letting on.   

"I have to disappoint you - I don't have one word or secret that describes the success that I had this year," he said on Sunday.

"Many different factors have happened in order for me to be where I am at this stage. I'm 28 years old. There is a reason why I've managed to reach the peak of my physical, mental abilities at this stage of my life."

Roger Federer and Novak Djokovic have emerged from the darkness, through the dry ice, and are about to toss a coin at the net and then warm up. 

The Serb blew some of his oxygen into his cupped hands before he was announced to the crowd. Cold inside the 02 perhaps? It'll soon be hotting up. 

If these mere words aren't enough (I am slightly offended), there's coverage on BBC Radio 5 live sports extra which you can listen to via the top of this page. 

Image source, get

Yeah, I know, incredible, isn't it. The reigning three-time champion has not lost a match since August. The man who defeated him? Roger Federer. (I know.)

There's more. The Serb has not lost indoors since 2012, which equates to a 38-match winning streak under canvas. His last defeat? By Sam Querrey in the second round of the Paris Masters.

Image source, BBC Sport

Piers Newbery
BBC Sport at the O2 Arena in London

Federer and Djokovic were not exactly best buddies in the formative years of their rivalry, but there is nothing better for a relationship than quality time spent together (I'm told). They head into their 43rd meeting with the score tied at 21-21, and Djokovic has the chance to move ahead in their personal duel for the first time since their initial meeting in Monte Carlo nine years ago. If the Serb required extra motivation, that will certainly provide it.

Something has got to give, as they say. 

Tonight will be the 43rd time these tennis greats go head-on into battle and victory will ensure one of them gains supremacy in the series. Admittedly, that won't be at the forefront of their minds when they warm up in the O2. 

Incidentally, Djokovic has never been ahead in the series. The Serb tied it at 21-21 with his dramatic four-set win in the US Open final in September. 

Hello! Welcome to live coverage of the ATP World Tour Finals bout between the world's best player, Novak Djokovic, and the world's third best, Roger Federer. 

Djokovic is a man unbeaten since August, a man looking to become the first player to win this title four years on the trot. He's irrepressible indoors. 

Federer... well, no man has won this tournament on more occasions (6), no man has won more matches in this tournament (49) and no man has won more prize money ($13,433,000) than the Swiss. 

Exciting, isn't it? 

Invincible? Unstoppable? Untouchable? 

How do you beat a man who has forgotten how to lose? When the odds are stacked against you, do you still believe? 

When there's no sign of weakness, no chink in the armory, no hint of letting go, can you still envisage victory? 

Of course you can! You're Roger Federer! A winner of 17 Grand Slam titles! (Sorry about the exclamation marks...)

Image source, Getty Images