World number 13 & 4-time Australian Open quarter-finalist Kei Nishikori has pulled out of the year's first Grand Slam and the inaugural ATP Cup. Nishikori is struggling to recover from his elbow injury. The 30-year-old, who has slipped to number 13 on World Ranking, has been out of action since the US Open as he aims to bounce back from his injury. Yoshihito Nishioka will replace him as the top-ranked player in Japan’s ATP Cup team in Perth.