Novak Djokovic finished off the regular ATP season with his successful title defence in Paris last weekend, but it's not quite all over yet. The top eight ranked players and doubles teams have one more tournament to play, and will convene at London's O2 Arena where they will battle it out to claim the ATP World Tour's most prestigious title. The World Tour Finals always finishes off the ATP season in style, and this year promises to be no different.
Federer joins Djokovic in group Stan Smith
Group Stan Smith sees 6-time champion Roger Federer join 3-time defending champion Novak Djokovic, along with Tomas Berdych and Kei Nishikori. Djokovic has to be the favourite to progress from the group, and although it looks like Federer will be joining him, Nishikori and Berdych are more than capable of overcoming the world number three.
While there are the usual doubts about Nishikori's fitness, Federer's greater inconsistency as he approaches the end of his career, and Berdych's resilience on the biggest stages, there seems to be nothing holding Djokovic back. Riding a 22-match winning streak, and at a tournament he's won every year since 2011, the Serbian should have every confidence in his ability to qualify from the group.
Murray heads group Ilie Nastase
World number two Andy Murray is the top seed in the other group, and he will do battle with Stan Wawrinka, Rafael Nadal, and David Ferrer for a place in the last four. Murray's consistency this year would seem to make him a good bet to qualify, although Wawrinka is unbeatable when he finds his form and Nadal has shown glimpses of his old self in the last few weeks.
Ferrer's past record seems to suggest that he's unlikely to progress, but it would be no surprise to see any combination of the other three into the semifinals. Perhaps the most likely two are Murray and Wawrinka - Nadal is not yet fully past his troubles this year, and this is a court which has never particularly suited him.
Top doubles team to be decided
Rather unusually, we arrive at the World Tour Finals without the Bryan brothers having sown up the top ranking. Marcelo Melo is riding high at No.1, while there are four teams - including Jamie Murray and John Peers - still in with a chance of being crowned the best team of 2015.
Despite claiming six titles, this has been the first year since 2004 without a Grand Slam title for the American twins, and the first time since Nenad Zimonjic managed it in 2008 that they will not finish the year as world number ones.
The 16-time Grand Slam champions will be looking to finish the season strongly to stand as the top-ranked team - at 37, they will not have many more chances to do so.
Djokovic favourite for fourth consecutive title
It's tempting to say that the electric atmosphere in the O2 arena, combined with the fast courts and the big occasion, make the tournament anyone's once we reach the semifinal stage. However, Djokovic's dominance has been so complete this year that it is tricky to see past the runaway world number one when considering possible titlists.
With almost 7000 more ranking points than his nearest challenger, Murray, and a stunning win-loss record of 78-5, the Serbian is the clear favourite for the title, even in such a tough tournament. Either he'll win his fourth in a row to cap off a spectacular season, or some other player will give the rest of the field hope for 2016. 2015 has indisputibly belonged to Djokovic.