Novak Djokovic continues his quest to regain the world No 1 ranking with a World Tour Finals semi-final showdown against Kei Nishikori. The Serb saw his 122-week grasp at the top of the rankings ended by Andy Murray in Paris almost a fortnight ago, but he can reclaim the position depending on the outcome this week.First he must overcome Japans Nishikori and with a dominant 10-2 record in the head to head he will start Saturdays semi-final in confident fashion, especially after a week that has seen him return to somewhere close to top form. Novak Djokovic and Kei Nishikori have met in two finals this year - on Saturday they meet for a place in the World Tour Finals showpiece Djokovic lost his opening set of the week to Dominic Thiem last Sunday but recovered to win the match in three sets and followed up with another pair of victories over Milos Raonic and alternate David Goffin,He had to come through an epic content against Raonic, eventually prevailing 7-6 7-6 while he had little trouble against late replacement Goffin who was dispatched 6-1 6-2.Nishikoris last four place had already been assured after Murrays straight sets win over Stan Wawrinka earlier in the day. Having begun the week with a superb victory over Wawrinka, Nishikori lost his next two matches.Having pushed Andy Murray close in his second match before losing in three sets, he ended the group stages with another three-set defeat, losing to Marin Cilic in three sets on Friday night. World Tour Finals - Story So Far Novak Djokovic Kei Nishikori beat Dominic Thiem 6-7 6-0 6-2 beat Stan Wawrinka 6-2 6-3 beat Milos Raonic 7-6 7-6 lost Andy Murray 7-6 4-6 4-6 beat David Goffin 6-1 6-2 lost Marin Cilic 6-4 2-6 3-6 John McEnroe talks about the price of ending the year as the world No 1 While world No 5 Nishikori has been in fantastic form this season, reaching his second and third Masters finals as well as his second Grand Slam semi-final, he will have to overcome a run of form against Djokovic that has seen him lose his last nine matches against the Serb.Djokovic has won all five of their meetings this year, including victory in both the Canada and Miami Masters finals, while Nishikori has won just three sets in their contests since his last win, a memorable US Open semi-final success in 2014. Kei Nishikori has come as close as anyone to beating Andy Murray this week, losing in three sets in the group stage The pair have twice met at the O2, Djokovic winning in the group stage in both 2014 and last year where he was a 6-1 6-1 victor.The signs are ominous for Nishikori but the 26-year-old appears to be relishing the challenge against the very best these days and a win would see him beat his semi-final performance of his World Tour Finals debut a couple of years ago. Djokovic v Nishikori met in the Miami final earlier this year Djokovic v Nishikori met in the Miami final earlier this year As ever it will be a doubles match which kicks off the evening and after confirmation of their world No 1 ranking, Jamie Murray and Bruno Soares will hope to book a final place when they face Rajeev Ram and Raven Klaasen.Murrays position as the year-end No 1 was confirmed when the French pairing of Nicolas Mahut and Pierre-Hugues Hubert lost their third group game of the week on Friday.Murray and Soares claimed the Australian and US Open titles and have won two of the three matches the pair have played.Follow all the action from the O2 on Saturday when the action begins on Sky Sports 3 from 12pm and 4pm or via our live blog here at www.skysports.com/tennis. Also See: World No 1 permutations Djokovic hammers Goffin Cheap Nmd Australia . 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