It will be remembered as Rafael Nadal’s year, but Novak Djokovic showed here on Monday night that the Spaniard will have to fight every inch of the way if he is to maintain his global supremacy through 2014.

The 39th meeting between the world's top two players ended in victory for the Serb as Djokovic completed a successful defence of his title at the Barclays ATP World Tour Finals, winning 6-3, 6-4 in an hour and 35 minutes.

Since losing to Nadal in the US Open final two months ago Djokovic has gone about his work like a man on a mission. Losing the world No 1 ranking to Nadal last month was inevitable given the Spaniard's extraordinary year, but Djokovic has been relentless in his refusal to let his rival draw clear at the top.

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Since that defeat in New York Djokovic has won 22 matches in succession, claiming titles in Beijing, Shanghai, Paris and now London. Tomas Berdych and company face a mighty task as the Czech Republic attempt to deny Djokovic the perfect end to the season when they face Serbia in the Davis Cup final in Belgrade this weekend.

The rivalry between Nadal and Roger Federer used to be the sport's headline act, but the Nadal and Djokovic show has taken over as the ultimate confrontation in men's tennis. They have already met more times than any other pair of players in the Open era and their tally of 19 meetings in finals is just one short of the record held by Nadal and Federer and by Ivan Lendl and John McEnroe. This result puts Djokovic 10-9 ahead in terms of victories in those finals and leaves the pair level at 3-3 in head-to-head confrontations this year.

If the year-end championships remain a jewel missing from Nadal's crown, there should be no underestimating the size of his achievement here this week in conditions which are the most challenging to his game; the world No 1 has not won a title on an indoor court for eight years.

Since returning in February after a seven-month absence with injury, Nadal has reached 14 finals and won 10 titles in 17 tournaments played. In recent weeks it seemed he might be winding down after his astonishing season, but over the last week he has given everything in his pursuit of this title. He will return to his home in Majorca more than satisfied after four victories over top 10 opponents here, even if the ultimate prize eluded him.

If the attraction of Federer and Nadal was their clash of styles, the appeal of Nadal and Djokovic lies largely in their stunning athleticism. Neither man ever admits that a cause is lost as they chase balls to every corner of the court, regularly turning stonewall defence into thrilling attack.

Djokovic, who generally has the advantage on hard courts, came out shooting from the hip, hitting the ball with great power and attacking at every opportunity. The Serb won nine of the first 11 points and soon led 3-0, only for Nadal to level at 3-3 by taking advantage of some careless hitting by his opponent.

However, when Nadal served at 3-4 Djokovic broke again after a rally that summed up his athletic brilliance. On break point Nadal seemed to be completely in charge as Djokovic chased a ball wide to his forehand, but the world No 2 hoisted a sensational lob into the far corner and went on to hit a volley winner. After 44 minutes Djokovic took the set, completing the job with an ace.

Nadal dug himself out of trouble in the opening game of the second set, but two games later he was broken again, hitting a forehand long on Djokovic's third break point. The Serb's consistently excellent hitting had Nadal at full stretch throughout. Nadal saved two match points before Djokovic completed his victory when the Spaniard hit a forehand wide. Djokovic's joy was unconfined – and his own season is not over yet.