Novak Djokovic
Djokovic extended his unbeaten run to 23 matches by sweeping Nishikori aside. Getty Images

Three-time defending champion Novak Djokovic sent an ominous message to his rivals in his opening match at the ATP World Tour Finals with a routine win over Kei Nishikori at the O2 Arena in London. The world number one began his pursuit of a fifth career title at the season ending tour finals by easing to a 6-1 6-1 win in 65 minutes.

The Serbian was dominant throughout claiming five breaks of serve as Nishikori toiled on his second appearance in the top eight shoot-out, which leaves the former US open finalist already struggling to reach the knock-out phase. Djokovic meanwhile remains in imperious form and faces either Roger Federer or Tomas Berdych in his second match on Tuesday 17 November.

Despite losing to Nishikori at the 2014 US Open, Djokovic's unbeaten run stretching back to August meant he entered his opening round robin match as the outstanding favourite to once again rubberstamp a dominant year with victory in the English capital. Nishikori had broken new ground in reaching last year's season-ending tour finals and though his 2015 had been disrupted by injury matching his achievements in reaching the semi-final would represent a decent end to the campaign.

Typically, Djokovic made a fast start to his title defence, breaking Nishikori in his opening service game to storm into a 3-0 lead. The Japanese right-hander did get on the board after surviving another break point via game-clinching forehand which produced an applause from Djokovic – though that would be the only moment of weakness from the number one seed as he took the opening set.

Nishikori took Djokovic to three sets in last year's semi-final, but any drama in their eighth career meeting was sucked out of the contest as the four-time champion broke early in the second set. A clinical forehand did put Nishikori on the board to the delight of a crowd which was growing increasingly concerned at the one-sided nature of the contest.

Having stayed competitive for long periods, the errors now started to flow from Nishikori's racket and when serving to stay in the match the number eight seed crumbled under the pressure. A double-fault allowed Djokovic a foothold and a backhand volley at the net completed the routine win.