Novak Djokovic and Kei Nishikori in action at ATP World Tour Finals

The ATP World Tour Finals are finally here and there's a great opening day in store at The O2 in London.

The star-studded event gets underway on Sunday afternoon with world No 1 and reigning three-time champion Novak Djokovic taking on No 8 seed Kei Nishikori.

You can watch the opening match of the event from midday on Sky Sports 3 HD.

It appears an uphill task for Nishikori; Djokovic carries a 22-match winning streak into the opening match of the season-ending finale. Not only that, but Nishikori is 4-2 down in their head-to-head match-ups on the ATP Tour.

Nishikori's 6-4 1-6 7-6 (7-4) 6-3 triumph over the Serb in the US Open semi-finals last year will certainly give him confidence going into this one.

But Djokovic has been in ruthless form this year - securing three of the four majors on offer - and has not been beaten indoors since 2012.

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Elsewhere in the Stan Smith Group, six-time champion Roger Federer and world No 6 Tomas Berdych meet on Sunday night.  

The Swiss player leads their ATP head-to-head 14-6 and comfortably won their two matches in 2015 - in the Rome and Indian Wells quarter-finals. Berdych last beat the 34-year-old in Dubai in 2013.

You can watch the first day of the ATP World Tour finals from midday on Sky Sports 3 HD.

Live Tennis: ATP World Tour Finals