The long season will finally conclude next week with the top eight players in the world competing at the ATP World Tour Finals.
Here's look at how the four players in Group A might fare at the O2 Arena in London. (For a breakdown of Group B, click here.)
1. Novak Djokovic
2015 record: 78-5, 10 titles
Season highlights: With his titles at the Australian Open, Wimbledon and the US Open, Djokovic reached double digits for Slam trophies. He's now in that elite, special stratosphere. Now speculation has intensified as to whether Djokovic, still just 28, has it in him to surpass Rafa Nadal's collection of 14 majors and possibly even Roger Federer's 17 Slams.
This isn't the first time Djokovic has won three majors in a year. He also did so in 2011, but this year he has been more dominant. He joined Rod Laver and Roger Federer as the only players in the Open era to make all four major finals.
Ironically, his loss in the Roland Garros final elicited one of the more uplifting experiences of his career. True, Djokovic would have accomplished the career Grand Slam had he won, but have you ever seen him as emotional and tearful on a tennis court as he was when he listened to the appreciation of the toughest crowd in tennis, the audience on Court Philippe Chatrier?
Season lowlights: Can losing in a Masters-level final ever be described as a lowlight? If it can, then his loss to Federer in Cincinnati, where the Swiss first attempted his "Sneak Attack by Roger," was a lowlight. Djokovic has now lost the Cincinnati final five times. If Djokovic can win that tournament, he would become the first singles player to achieve the Career Golden Masters, which is a complete set of the nine Masters-level tournaments.
What to expect in London: Can Djokovic become the first man to win four successive titles at the season-ending tournament? It's a rhetorical question, of course. The champion in Shanghai in 2008 and in London for the past three years, the Serbian is attempting to score the title for the fifth time. That would put him in joint second place on the tournament leaderboard, alongside Ivan Lendl and Pete Sampras, and just one title behind Federer.
3. Roger Federer
2015 record: 59-10, six titles
Season highlights: As has always been the case with Federer, it's not just what he accomplishes, but how he accomplishes it. Federer's run to the Wimbledon final came courtesy of a near-perfect performance against Murray. Later that summer, Federer's SABR strategy not only created a massive buzz but helped him beat Djokovic in the Cincinnati final.
Federer then reached the US Open final for the first time in six years. Unfortunately for Federer, his attempt to win an 18th Grand Slam was twice denied by Djokovic, in London and then in New York. At Wimbledon, Federer simply didn't play as well against the Serbian as he had done against Murray a round earlier, while at the US Open, Federer didn't make the most of the copious breakpoint opportunities he created.
To date, Federer has six titles, only the second time since 2007 he's reached that mark.
Season lowlights: Federer failed to make the semifinals or better at the Australian Open for the first time since 2003 after Andreas Seppi beat him in the third round.
What to expect in London: Plenty. No one else has Federer's experience in the year-enders. This is the 14th straight season Federer has qualified for the season-ending tournament, which is a record. (Andre Agassi also qualified 14 times, though not consecutively.) Six times Federer has won this tournament, which is another record, with two of those titles coming in London.
Last season, you might recall, Federer withdrew moments before the final against Djokovic, citing back pain. You have to imagine he wants nothing more than a seventh title.
6. Tomas Berdych
2015 record: 57-19, two titles
Season highlights: Nobody, but nobody, beats Berdych 18 times in a row. What a cathartic win it was for the Czech when he finally got the best of Rafael Nadal in the quarterfinals of the Australian Open. After dropping 17 in a row to the Majorcan, Berdych couldn't hold back his elation in Melbourne.
Berdych's parlayed that win into strong showings in Doha, Rotterdam and Monte Carlo, where he made the final of each. Since the US Open, Berdych won in Shenzhen and Stockholm, his first two titles of the season.
Season lowlights: Well, after a promising win against Nadal in Melbourne, things got a little squirrely. There was much debate about Dani Vallverdu, Murray's longtime hitting partner and assistant coach who recently joined Berdych's team. Tensions rose to the point where Kim Sears, Murray's then fiancé, was caught on TV cussing at the Czech.
Berdych lost that match and to date has yet to win a major title. Has anybody suffered more than Berdych during an era dominated by some of the greatest players in the history of the sport?
What to expect in London: This is the sixth time Berdych has qualified for the season finale, but only once has he progressed past the group stage. However, Berdych has been playing solid tennis in recent weeks, and he has enough power in his game to trouble anyone.
8. Kei Nishikori
2015 record: 53-14, three titles
Season highlights: After last season's history-making run in New York, where he became the first Asian man to appear in a Grand Slam singles final, Nishikori hasn't made the same splash in 2015.
Even so, this has been a solid year from the Florida-based player, who has gathered titles on two different surfaces (hard and clay) and on both sides of the Atlantic. He also went deep at a couple of Grand Slams, making the quarterfinals of the Australian Open and Roland Garros.
Season lowlights: A year after making the final at Flushing Meadows and finishing as the runner-up to Marin Cilic, Nishikori was beaten in the opening round, when he lost a five-setter to Frenchman Benoit Paire, a result that made him feel "very sad."
That hadn't happened since Agassi, a beaten finalist at the 1990 US Open, lost in the first round in New York the following summer. Twice in 2015, Nishikori departed a Grand Slam before it had really got going, as he withdrew from a second-round match at Wimbledon because of a leg injury.
What to expect in London: By mid-November, no tennis player is feeling his perkiest, but there is greater doubt over Nishikori's physical capability as he prepares to play this tournament for the second successive year. Having withdrawn from a tournament in Basel because of a sore shoulder, Nishikori retired from a match in Paris against Richard Gasquet after feeling pain in his abdominal muscles.
Still, Nishikori is more than capable of playing some accomplished tennis at this tournament. He handed eventual winner Djokovic his only loss of set in last year's event in their semifinal meeting.