THIS afternoon, in the modern, hi-tech surroundings of a cosily comfortable arena, two young friends will march out to play each other in the entirely civilised sport of tennis.

Then, in the professional argot, they will proceed without hesitation to beat each others' brains out. More than 17,000 fans inside the O2 Arena will cheer wildly. A worldwide television audience of millions will sit entranced by every rally, moved by every moment of brilliance and stunned by the odd error forced by fatigue or mental pressure.

It may all take some time.

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Novak Djokovic, the world No.1, and Andy Murray, the world No.3, face each other for the seventh time this year, this latest confrontation coming in Group A of the Barclays ATP World Tour Finals. The Serb leads the Scot 9-7 in the career head-to-head but the six matches this year have been split down the middle, with Murray having cause to believe he has had the better of his old mate. The Scot lost in the semi-final of the Australian Open but defeated Djokovic en route to an Olympic gold and in the final of the US Open.

The most revealing statistic of all, though, is that each set they have played over the year has taken about an hour. Thus, if Murray and Djokovic play to form in terms of time, the match this afternoon could last as long as two football matches.

This premium on endurance has been one of the most obvious factors of an emerging rivalry that many believe will dominate the world game. Pat Cash, the former Wimbledon champion, has pointed out that three-set matches in his day were over in two hours but the changes in surfaces and in equipment have helped make Murray and Djokovic marathon men on court.

The length and intensity of the matches pay testimony to the extraordinary fitness of both these players. Murray admitted inspiration is not difficult to find. His fitness has been honed by his craving for a major title and his early realisation that he had to become stronger and more resilient on court.

He has also met some interesting characters along the way, not least this week when he spent time with Jason Arday. "He has epilepsy but ran 30 marathons in 35 days," said Murray. "He told me that on the 20th day he fell – he had a seizure – and had a hairline fracture in his leg but carried on. He had asked to meet me and that was pretty cool speaking to someone like that."

The serial marathon runner was "pretty pumped" after meeting Murray and his trip back to the centre of London from Greenwich on a boat where other passengers included Juan Martin Del Potro and the Bryan brothers.

The meeting made an impression on Murray, who is prepared yet again for a marathon with Djokovic this afternoon. At 25, the players have known and played against each other for 14 years, but now their contests are exclusively for the big prizes and at the business end of tournaments.

They will both toil under the roof of the O2 where the packed arena produces high temperatures, but Murray is looking forward to the atmosphere. "It was warm but that is the way it should be. There are so many people in there and it makes a difference to how the court plays as well," he said.

"It was much easier to move than in the last few days when there was no-one in there," he said of practice sessions, one of which was with Djokovic on Saturday. "It was cold, it was sore on the joints and stuff and you get no energy off the court. I hope it is warm the next few days."

Murray, amazingly, has only played one indoor tournament – Paris last week – this season but admits he loves the surface and is relishing the challenge in London.

The defeat of Tomas Berdych has given the Scot an encouraging start but now Djokovic, who won in straight sets against Jo-Wilfried Tsonga, stands in the way of almost certain automatic qualification for the semi-finals.

Looking back on a season that subsequently produced his first grand slam, Murray said of the Olympic semi-final between the pair: "It's tough to know which match turned things or which match helped the most, but that was a big match for me. It wasn't necessarily an unbelievable standard, but both of us had a lot of chances. I also know how much all the players want to do well at the Olympics, so to beat him there was big."

Djokovic, meanwhile, was coy about the health problems he encountered last week in Paris and the condition of his father, who was in hospital with a respiratory condition.

He accepted, though, that he would have to gather all his resources for what could be another marathon today. "I need to be ready for long rallies and a physically demanding match," Djokovic said.

That much is certain as both players vie to be last man standing in the most gruelling of rivalries.

* Those studying the form in the hope of picking a likely winner at the Barclays ATP World Tour Finals may not stray too far from the genuine thoroughbred in the field, the perennial course and distance winner, writes Hugh MacDonald.

Roger Federer, at 31, is routinely accused of being on his last legs but simply rebuts these scurrilous allegations with performances of class and conviction.

Federer is a strong favourite to qualify from Group B, not least because his combined record against his three opponents in the group rose to 32 victories to three defeats after a gentle workout against Janko Tipsarevic of Serbia yesterday.

If Andy Murray and Novak Djokovic are contesting the Group of Death, Federer may be safely considered to be in the Group of a Wee Chill That Will Soon Pass. The Swiss took one hour and eight minutes to beat Tipsarevic 6-3, 6-1 and take his career record against the Serb to 6-0. The No.8 seed said he was suffering from a cold but. with wonderful honesty, admitted: "I probably would have lost anyway."

Most people do against Federer in any tournament but this was his 40th victory in his 47th match in a personal tournament history stretching back to 2002 and which includes six titles.

Federer remembered his first venture into the tournament – he beat Juan Carlos Ferrero – and is aware that many of his contemporaries, including the Spaniard and such as Andy Roddick, are now retiring, leaving him the elder statesman at the very top of the world game. "There is a life after tennis; there must be," he said, almost in a desperate hope.

Federer is still consumed by the sport and he now must shrug off the challenges of David Ferrer (he leads the Spaniard 13-0 in head to head) and Juan Martin Del Potro (13-3 to the Swiss). The Argentine, though, can claim that his three victories against Federer carry a heavy significance. He beat the 17-time grand slam winner twice in 2009: once at the tour finals in London and the other in the final of the US Open. Del Potro was forced to wait three years before recording his third triumph; he beat Federer last month in Basle.

Ferrer later extended his indoor winning streak to 11 matches with a 6-3, 3-6, 6-4 defeat of Del Potro and will be next to take on Federer tomorrow.

Ferrer is the form player in the field after winning titles in Valencia and Paris, his first Masters crown. He has also won more matches and more titles on the ATP Tour than any other player and has the chance to overtake his compatriot Rafael Nadal as world No.4 if he can reach the final here.

Federer, though, is long odds-on to qualify from the group and played beautifully against an opponent who caused him only minor difficulties. Asked mischievously what he could not do in life, Federer replied: "I can't cook. There's many things I can't do that I wish I could do. I can't skate." He can do the cakewalk, though.

* England's Jonny Marray and his Danish partner Freddie Nielsen, the Wimbledon doubles champions, won their first match in the tour finals, saving two match points to beat the fifth-seeded Indians Mahesh Bhupathi and Rohan Bopanna 6-4, 6-7 (1-7), 12-10. Marray, though, confirmed he was still on the lookout for a permanent doubles partner for next season as Neilsen is committed to playing singles.