The Japanese player reached the quarter-finals of last year's Melbourne tournament, losing to Stan Wawrinka, but a promising start to the campaign was soon blighted by injuries.
Nishikori also advanced to the quarters at the French Open before he was beaten by Jo-Wilfried Tsonga, but his season then went on a downward spiral
The 26-year-old withdrew from Wimbledon at the second round stage due to a calf injury, while he suffered a first-round exit at the hands of Benoit Paire at the US Open, after suffering a hip injury in August.
But Nishikori did give glimpses of his talent, capturing his third title of the year at the Citi Open, and also defeated Rafael Nadal at the Rogers Cup.
He qualified for the season-ending ATP World Tour finals and failed to progress from the group stage, despite a victory over sixth seed Tomas Berdych.