Djokovic's historic moment, Murray's final mission

The ATP World Tour Finals have come and gone. Once again, the theme was an all-too-familiar one with Novak Djokovic winning another tournament, his fourth straight year-end title.

After a sensational week in London, and with a historic Davis Cup final coming up, our writers have a few thoughts to share:

Showing gratitude

Nic Atkin, ESPN UK: It's a long, grueling season for those playing on the ATP Tour. But it's also a slog for a lot of the media, who are there week in, week out, traveling the globe to follow these players around. It's nice to see some of the players recognize this.

"Have a good Christmas and Happy New Year," Rafael Nadal said to the journos, calling them back as they had started to file out at the end of his final news conference in London.

All this despite looking fatigued and fed up after the crushing disappointment of his semifinal defeat to Djokovic.

Djokovic then carried on his tradition of handing out chocolates from his native Serbia to all those packed into the main interview room after he had won the title Sunday. A Japanese journalist said "thank you" to Djokovic after taking one from the box. Djokovic replied "arigato" in kind.

After two boxes had run out, Djokovic even handed his own personal energy bar to a journalist who had missed out on the other treats. A touch of class off the court from someone who has shown so much on it this season.

Davis Cup tension

Tristan Barclay, ESPN UK: Despite guiding Britain to its first Davis Cup final since 1978, it seems Andy Murray still has a point to prove in some quarters if this weekend's clash with Belgium is to be played.

The world No. 2 has been accused by former British captain David Lloyd of failing to inspire a future generation of tennis stars. Lloyd claimed that Murray's pair of Grand Slam victories has not been enough to convince kids in the UK to pick up a racket rather than a soccer ball. Winning tournaments, Lloyd said, is no substitute for putting your heart and soul into grassroots tennis.

Posting on Instagram last Sunday, Murray responded, "Thanks for all the messages of support on social media today and for seeing [Saturday's] criticism for exactly what it was #bitter #backthebrits."

Former players have jumped to Murray's defense, too, with former British No.1 Tim Henman saying others have to take advantage of the interest Murray creates in the game.

Murray mailing it in?

Peter Bodo, ESPN.com: "Did he or didn't he?"

That's the big question regarding Murray and his failure in the late stages of the ATP World Tour Finals. Full question being: "Did Murray let his two final round-robin matches get away from him because of Great Britain's critical Davis Cup final at Belgium next weekend?"

Murray denied that the pending tie in Ghent, Belgium, was a distraction following his straight-set loss to Stan Wawrinka in the third round of round-robin play -- a loss that eliminated Murray from contention. (Murray had been crushed in his previous match by Nadal.)

"I wanted to try to win the tournament," Murray insisted in his postmatch news conference. "Obviously you have to try your best right way through to the end. It wasn't enough."

On the other hand, Murray has been practicing on clay for the Davis Cup final right up and even during the World Tour Finals, and two weeks before the London shootout of the top eight ATP players, he said he might skip the year-enders entirely in order focus on Davis Cup. It obviously means a great deal to Murray.

Great Britain hasn't hoisted the Davis Cup since Fred Perry led his squad to triumph in 1936. In other words, this is like Murray winning Wimbledon all over again -- a great feat for a once-great tennis nation that has experienced a long and frustrating dry spell.

Of course, Murray toying with the idea of bypassing the tour's showcase finale meant a great deal to the ATP. You can bet the telephone lines lit up. So Murray did the right thing. He fulfilled his ATP obligation and played, recorded a nice win over David Ferrer, then took his two losses and got the weekend off.

It would be unfair to question Murray's honesty or integrity. But this is a guy who locked down a career-high ranking of No. 2 this year. It's impossible to imagine that the Davis Cup wasn't on his mind.

The best rivalry in tennis is ...

Matt Wilansky, ESPN.com: The overarching -- make that obvious -- storyline on the men's tour this year was Djokovic. There's no debating that. But there is some dispute as to what the greatest rivalry in tennis is.

With the ATP World Tour Finals completed, Djokovic is now 23-23 lifetime against Nadal, having beaten the Spaniard in the London semifinals, and 22-22 versus Roger Federer. (They split two matches this past week.) Those are the two most frequent matchups in the Open era.

So ... which is the better rivalry?

If the present means anything, Djokovic and Federer played eight times in 2015, with the Serb snaring five of those wins. However, the only real drama-laden match came at the All England Club, where Djokovic won a five-set thriller in the championship match (the second year in a row he thwarted Fed in the fifth set of the Wimby final).

Djokovic and Nadal went head-to-head four times this season, and the Serb swept every set. However, these two have a grueling history.

At the 2009 Madrid Masters, Djokovic edged out Nadal in a 4-hour, 9-minute three-setter, the longest Masters match ever. And, of course, there was the 5½-hour Aussie Open final three years ago and the epic semifinal clash that Nadal won at the 2013 French Open.

It's exhausting just thinking about it.

Perhaps Djokovic himself can distinguish the two rivalries.

"In terms of matches played, maybe the most exciting matches that I've played maybe the Nadal rivalry would be the one I would pick," Djokovic told the media after Sunday's title. "Again, two different rivalries because two different players.

"Those two rivalries made me a better player, the player I am today, no doubt. Made me understand what I need to do both on and off the court to be able to be in this position now."

Djoker-Federer or Djoker-Nadal. We could parse the numbers for days and not come up with a definitive answer.

Let's just hope these two rivalries don't end anytime soon.