Murray with an early dropper, but Nadal is quickly onto his toes, sniffing it out and forcing the Scot to drill a backhand into the tape. 15-0. 

Nadal, apparently, according to a colleague to the left of me, has the squeakiest shoes in tennis. Admittedly, it does sound as if he's being serenaded by chipmunks every time he moves side to side. 

Murray sneaking ahead at 30-15, watched on from the sidelines by John McEnroe, Lleyton Hewitt and a number of other greats. The Scot lassoes a forehand, which catches Nadal cold and the Spaniard is two break points adrift. Murray, calmly, approaches the net and volleys his way home. 

Image source, Reuters

The warm-up is over. Finito. The lights have been dimmed other than the spotlights which are at full wattage illuminating the azure court. Rafael Nadal to serve first. Let's see what he's got. 

Image source, Getty Images

Tim Henman
Former British number one on BBC Two

Rafael Nadal already has 14 of his sport's biggest prizes, but can the Spaniard win another Grand Slam? That is the question Sue Barker has just posed on our screens. Another French Open title before retirement is surely a given? Who knows...

Tweet your thoughts using the hashtag #bbctennis. 

Andy Murray has been talking about impending fatherhood and next week's Davis Cup final against Belgium in his latest BBC Sport column. 

He has also spoken about his straight-sets win over David Ferrer on Monday. 

"It was a little bit off for a couple of games in my first match on Monday and my returning could have been better, but to beat a guy like David Ferrer in straight sets is always encouraging and physically I felt good, so I'm happy about that," he says. 

Image source, BBC Sport

We just assume Nadal has won it all - a career Grand Slam, Olympic gold, a Davis Cup title - but the ATP World Tour Finals title has, so far, eluded him. 

The Spaniard has appeared in these season-ending spectacles on six occasions, but the best he's done is finish runner-up in 2010 and 2013.

A title in 2015 would be a shock, but stranger things have happened in sport. 

Image source, BBC Sport

How have these tennis big cheeses performed against each other in the past? 

Rafael Nadal holds a supreme 15-6 advantage in their head-to-heads, but Andy Murray thrashed the Spaniard when they last met, beating the best clay court player in history on his favoured surface. 

The Scot was flawless as he secured a 6-3 6-2 win in Madrid, which is why the consistent Brit is the favourite this afternoon.

Image source, BBC Sport

The master of ceremonies is warbling, rolling the Rs as he prepares to introduce R-r-r-r-rafael Nadal to the audience. 

They don't seem to be running low of dry ice at the 02 just yet, the Spaniard having to walk through thick fog before reaching the court.

Andy Murray is the next to receive a gladiatorial call. The Scot waves to his fans, of which there are plenty, as he walks towards his chair. A warm reception for both. 

Image source, AFP/GETTY

Image source, Getty Images

Piers Newbery
BBC Sport at the O2 Arena in London

"So if Murray wins, and Ferrer wins, Murray is through.... right?" It's that time of the year again, when the word 'scenarios' strikes fear into a tennis journalist's heart. 

Crumpled pieces of ATP paper detailing the different possible outcomes from today's matches are being passed around the media room, with the dreaded 'percentage of games won' tie-breaker potentially looming later in the week A straight-sets Murray win would make things simple for everyone. Well, simpler. Although....

Victory for Murray this afternoon will all but secure the Scot a spot in the semi-finals. (He will qualify if he beats Nadal and the evening match sees any result other than a straight-sets win for Stan Wawrinka.)

There's more because victory will also ensure Murray finishes the year as the world number two, which would be the highest year-end ranking of his career.

It's all about the number two.

Image source, BBC Sport

A bird's eye view of the world's best players...

There's good news. You can watch this match on all sorts of platforms. On one screen, a second screen, a third...the option is yours. 

There is, as always, live text commentary, which hopefully you know about, and there's live BBC TV and BBC Radio 5 live sports extra coverage at the top of this page from 14:00 GMT. 

You can find Sue Barker and co lurking in the 'live coverage' tab. 

Image source, Getty Images

Piers Newbery
BBC Sport at the O2 Arena in London

"Andy Murray takes on Rafael Nadal this afternoon knowing a victory would put him within touching distance of the last four at the ATP World Tour Finals.

"The Briton has been among the earliest arrivals at the O2 Arena this week as he looks to jump the queue of singles and doubles players and get good practice time on the centre court.

'There's no point in sort of practising at 50% intensity," said the Scot. "You need to kind of go out there and practice hard if you're going to spend less time on the court.'"

Image source, BBC Sport

Hello! Welcome to live coverage of the ATP World Tour Finals ding-dong between world number two Andy Murray and Rafael Nadal, the fifth best player in the world. 

Nadal has had a torrid year. Injuries, illness, sorry results. His 19 losses in 2015 is his most in a season during a 14-year career. He even dropped to 10th in the world in the summer. 

But the 29-year-old is improving and his straight-sets win over Stan Wawrinka in his opening match of this tournament was a hint that next year will be a better one for the 14-Grand Slam champion. 

There was a time when the outcome of this match would have been a foregone conclusion. In those days Rafael Nadal was King of the heap. He ruled the roost. But tennis' pecking order isn't as it once was. Nadal isn't what he once was. 

Image source, Getty Images