In recent years, no tennis player has been more physically fit than Andy Murray. Mental fitness, however -- particularly under duress -- has been an enduring issue.

For the better part of six years, Novak Djokovic has been the best player in the world, the consummate model of commitment and mental toughness.

And now, after Murray's decisive 6-3, 6-4 finals victory Sunday over Djokovic at the Barclays ATP World Tour Finals, they have traded places.

With his domination against Novak Djokovic at the World Tour Finals, Andy Murray became the 17th player to finish the season ranked No. 1. KIRKGLYN KIRK/AFP/Getty Images

With the year-end No. 1 ranking on the line in a single match, Murray absolutely commanded the court, dictating to Djokovic the way Djokovic dictated to him in winning 24 of 34 previous matches -- and 16 of the past 19.

In two of those matches, back in January in the Australian Open final and, four months later, in the final at Roland Garros, Djokovic was indomitable. But here, the roles were reversed.

"I just played very poorly, made a lot of unforced errors from the backhand side," Djokovic said to the media afterward. "On the other hand, credit to Andy for being mentally tough and playing the right shots, making me play extra shots in every rally. He definitely deserved to win."

Murray's forehand was massive, and so were both his first and second serves, but it was his composure that was so unnerving to the man across the net. Two points from the match, at 30-all, Murray fearlessly bounded up to the net and forced a long lob.

"It was obviously a big match, a very important win for me," Murray said later. "It was just a huge match to finish the year, to try and obviously finish No. 1. Obviously, this is a major event, as well, and one I've not done well in in the past.

"So it's been a great week."

Djokovic, the four-time defending champion, meanwhile, was a mess -- off balance and tentative.

Tellingly, he missed a bunny of an overhead in the sixth game of the first set and an even easier volley in the fourth game of the second. His cross-court backhand, the best and most natural shot in his entire palette, missed early and often.

Djokovic came in with an enormous edge in terms of freshness. He dropped only five games in his Friday and Saturday matches; Murray went 3 hours, 38 minutes in his Saturday semifinal win over Milos Raonic, his second three-hour-plus match of the tournament.

But fatigue was never a problem for Murray, whose body language was exemplary.

"I guess, even though he has had very long matches, especially the one yesterday, people were thinking maybe he's going to be slightly tired," Djokovic said. "But he didn't seem so. More than anything, he felt comfortable in the rallies and exactly knew what to do."

Murray's first big push to the top came four years ago. He won the gold medal at home in the London Olympics, and then, after losing his first four Grand Slam finals, he finally won one, the 2012 US Open, beating Djokovic in the final. The same scenario played out at Wimbledon the following year.

Ivan Lendl, who coached Murray through that best-ever stretch, gets a lot of credit for giving him the gift of confidence. Lendl, a no-nonsense, eight-time Grand Slam singles champion, implored the cautious Murray to aim for the lines -- and not to despair when he missed. Going for it, Murray came to learn, was the same thing as believing in yourself.

That surge might have eventually taken him to No. 1, but back surgery after the 2013 US Open and the departure of Lendl early the next year arrested that potential championship development.

In June, Lendl agreed to return to Murray's coaching box, but he has hardly been a fixture in the athlete's daily routine. Back in February, Murray and his wife, Kim, became parents. Pretty much ever since, outside of that French Open final and a five-set quarterfinal loss to Kei Nishikori at the US Open, things have gone swimmingly for Murray.

At the age of 29, he finds himself ranked No. 1 for the first time ever. Murray finished the season on a run of 24 consecutive match wins. This, after establishing a career-best of 22 straight victories earlier this year.

That's a ton of momentum going into 2017.

"I would like to try and stay there, obviously," Murray said of the No. 1 ranking. "It's taken a huge effort the last five, six months to get there. I'm aware that's going to be extremely difficult because I had a great year this year. I only managed to do it by one match. To repeat that again next year is going to be extremely difficult.

"But now that I've got there, I obviously would be motivated to try and stay in that position."