In the cat-and-mouse game of postmatch interviews in London, Novak Djokovic has been driven to testiness on several occasions when questioned closely. After defeating Milos Raonic in a round-robin match earlier this week, he was asked if there were times in the past when he felt unbeatable -- and if he now felt more vulnerable.

"Well, I never felt unbeatable, and I never will," Djokovic said. "First of all, it's not my mindset. Second, I don't think it's fair to say you're unbeatable. There are few athletes ever in their career that have retired unbeatable.

"I did have certain periods of my career, especially in the last five years, when I really felt very confident on the court, and I was dominating the play."

What about vulnerable?

For the first time in history, the final two players at the ATP World Tour Finals will be playing for the world No. 1 ranking. Getty Images

"I don't feel vulnerable," Djokovic snapped.

Heading into the last men's ATP match of the year, he looks like something approaching unbeatable. As difficult as it is to fathom given his second-half results, the No. 2-ranked Djokovic is a single victory from regaining the No. 1 ranking.

He meets old friend and recent nemesis Andy Murray in the championship final of the Barclays ATP World Tour Finals on Sunday at 1 p.m. ET on ESPN2 & WatchESPN. For the first-time in year-end history, the season's No.1 ranking will be decided in a winner-take-all final.

It would be an unprecedented fifth consecutive year-end title for the 29-year-old Serb in a year when he won at least seven titles for the fourth consecutive year and two of the four majors this year.

Does this sound like a "vulnerable" man?

Djokovic obliterated Kei Nishikori 6-1, 6-1 in Saturday's second semifinal. The match was over in a blazing 66 minutes and couldn't have been more different than the first semifinal.

That exquisite piece of performance art was a 5-7, 7-6 (5), 7-6 (9) Murray victory against Milos Raonic.

"Very glad, obviously, that I get to experience that [dominance] the last couple matches," Djokovic said in his on-court interview. "Pretty much flawless."

Murray, meanwhile, looked world-weary at times and saved a match point in that extraordinary third-set tiebreaker. The question must be asked: Does Murray have anything left for Djokovic?

Three days earlier, Murray and Nishikori played a 3-hour, 20-minute match -- the longest three-set match in the history of the year-end event. Saturday's epic ran a scintillating 3 hours, 38 minutes, which turns out to be the longest three-set match of the 2016 season.

That's a lot of mileage this close to the finish line.

"I don't know how I'll feel tomorrow," Murray said after the match. "Obviously, tired just now because it was a really hard match. It wasn't just that it was physically hard, it was mentally a tough match, too.

"It was pretty stressful."

Murray, who made the one-hour commute to his suburban home when he played day matches, will be staying at a Thames-side hotel. After the match, he immersed himself in an ice bath, stretched and had a massage from his physio. Murray said he'd go through another round of recovery and try to sleep in as late as possible. He's scheduled to practice at 2:30 local time, 3½ hours before the final.

Murray, riding an enormous wave of momentum, has won 22 consecutive matches, matching his career high set earlier this year.

But his personal on-court history with Djokovic -- they first met 18 years ago in junior tennis -- is not terribly pleasant. Djokovic has a commanding 24-10 record in their head-to-head matches, and upon closer examination, it's even more lopsided in recent years.

After Murray broke through with his first Grand slam singles title at the 2012 US Open, Djokovic held an 8-7 advantage. Since then, the Serb has won 16 of 19, including the finals of the two majors he's won this year, the Australian Open and French Open.

And there's this delicious symmetry: After beating Murray in four sets to win the long-sought-after career Grand Slam in Paris, Djokovic lost his way. Here, he has a chance to beat Murray again, finish the season on a breathtaking high and set himself up for the 2017 season.

Since the beginning of 2011, Djokovic has dominated men's tennis, winning 11 of 24 Grand Slam singles titles. For context, consider the other 13 major winners over the past half-dozen years: Rafael Nadal (5), Stan Wawrinka and Murray (3 each), and Roger Federer and Marin Cilic (1 each).

In that span, the only time Djokovic finished with the No. 2 year-end ranking was 2013, when Nadal won two Slams. Djokovic regained the top spot the following summer after winning Wimbledon, and for 122 weeks, he did not relinquish it. That glorious run ended two weeks ago in Paris, when Murray -- after slowly reeling Djokovic in following Roland Garros -- took the No. 1 ranking for the very first time.

In this year-end event that is without Federer and Nadal, it's appropriate these two are in the final with so much at stake. They are the best in men's tennis, and it seems likely they could continue their dominance for a few more years.

"Hopefully, we can have a good final," Djokovic told the pro-Murray crowd. "It's really a privilege to be part of this history with Andy. There are two of us on the court.

"Let's try to enjoy the occasion."