Andy Murray went into the match having played over three hours more than Novak Djokovic during the course of the week, but in the end it was his big-match sharpness that prevailed.
He played with far greater purpose than Djokovic, who made 30 unforced errors in an unusually erratic performance.
Murray’s experience of winning matches day in, day out through the second half of the year shone through, in contrast to the more tentative Djokovic we have seen since he won the French Open.
The Briton did open the match with a double fault, and another three points later, but he pressed for the break at 3-3 after Djokovic sent an easy smash wildly long and wide.
The breakthrough game two games later when Murray fired a forehand into the corner for a 5-3 lead, and the set followed after 46 minutes.
Murray simply grew stronger and Djokovic more error-prone as the second set unfolded, two breaks giving him a seemingly impregnable 4-1 lead, before the champion fought back.
Djokovic recovered one break and raced through a service game to cut the deficit to 4-3, but Murray steadied the ship with a solid service hold to move within a game of victory.
The victory also ends Djokovic’s four-year run of success at the tournament and the Serb’s bid to equal Roger Federer’s record of six titles.
Murray, who had won 10 of the pair’s previous 34 encounters, added: “We’ve played Grand Slam finals and in the Olympics before, but I am very happy to win.
“It is something I never expected,” he said, his win capping off a weekend that also saw brother Jamie Murray and partner Bruno Soares crowned world number one doubles pair.
“Andy is definitely number one in the world,” said Djokovic. “He deserved to win. He is the best player.
“In the decisive moments, I wasn’t able to come back. I played better late in the match but it wasn’t enough.” He added.
With the year-end top ranking on the line, Murray won the much-anticipated final 6-3 6-4 at London’s O2 Arena.
“I’m very happy to win and to be world number one is very special,” said the Scot. “It’s very special playing against Novak in a match like this.” Murray concluded.