Nishikori has endured a frustrating campaign over the last 11 months with a series of nagging injuries ruining his hopes of challenging for the four majors.
After making his Grand Slam breakthrough with a run to the final of the 2014 US Open, the 25-year-old came into this year tipped as the most likely challenger to the sport's big four of Novak Djokovic, Roger Federer, Andy Murray and Rafael Nadal.
But instead he was beaten in the quarter-finals of the Australian and French Opens and then pulled out of the Wimbledon second round due to a calf injury before suffering an embarrassing US Open first round defeat.
"I'm excited for next year. I think physically I have to be little more strong," Nishikori said after his season came to an end with a defeat against Federer at the ATP Tour Finals in London on Thursday.
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"I try to work on little things to improve a little more. Try to be healthy, I think that's most important."
It wasn't all doom and gloom for Nishikori in 2015 as he did win three titles -- in Memphis, Barcelona and Washington -- as well as maintaining his place in the world's top eight.
- Will to win-
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A less competitive player might have thrown in the towel after winning just two games against Djokovic.
However, Nishikori's ferocious will to win is one of his main assets and he recovered to defeat Tomas Berdych in his second group match.
Chasing a third career win over boyhood idol Federer, Nishikori broke the Swiss great five times and pushed him to the limits before succumbing to a 7-4, 4-6, 6-4 loss that confirmed his elimination.
"Plus I missed three Masters this year, but still I'm in the top 10. If I can do little better than this year, I think I can go little higher (in the rankings).
"I think I have to adjust little things to win those top players. But I think getting close."
One of Nishikori's main aims during his three-week training camp ahead of January's Australian Open is to improve his play at the net.