3 players who can dethrone Novak Djokovic at the ATP World Tour Finals

The ATP World Tour Finals have begun in London, with seven top-ranked players looking to unseat two-time defending champion Novak Djokovic.

The absence of Rafael Nadal due to an appendectomy opened the door for US Open champion Marin Cilic to enter the fray.

With round-robin action underway, here are three of the likeliest candidates to dethrone Djokovic.

Kei Nishikori

Nishikori reached his highest career ranking on the ATP Tour, sitting fifth going into the Finals. He captured four of his seven titles in 2014 and reached the US Open final.

Nishikori had to retire in his battle against Nadal at the Masters 1000 in Madrid earlier in the year, and lost to Roger Federer at the Gerry Weber Open. But he stormed back to capture four titles while nursing a leg injury to qualify for the tournament in October.

If Nishikori can ignore any issues that arise, he has proven with his breakout season he can steal this title from Djokovic.

Stan Wawrinka

After capturing his first-career Grand Slam title at the Australian Open over an ailing Nadal, Wawrinka captured another two titles en route to a No. 4 world ranking.

As well as helping Switzerland to their first Davis Cup final since 1992, he won his first Masters 1000 title at Monte Carlo in April. Wawrinka continued to have a great year, even after suffering his first loss to a player ranked out of the top 100 since 2013.

Wawrinka made Nadal look like an average player when he defeated him in January. The way he moves around the court is relatable to Federer and, at the age of 29, he remains in his prime - albeit at the very edge.

Andy Murray

Murray's 12-match winning streak was broken in October by none other than Djokovic at the Masters 1000 Paris. But Murray has his game back after a necessary back surgery ended his last season prematurely.

As well as adapting well after surgery, he picked up wins at the Valencia Open 500, Erste Bank Open, and Shenzhen Open. Two of the titles came in October, which bodes well for Murray.

A fun graphic from the National Post shows how each finalist has fared against their fellow opponents with a heat map of their head-to-head records.