"Result-wise this has been my best year. I've played well against the top-10 players and have been winning against the likes of (Rafa) Nadal and (Andy) Murray, who I couldn't beat before.
"I'm making fewer mistakes as I get older. I hope to keep building, bit-by-bit."
Nishikori started the campaign by reaching the quarter-finals of the Australian Open and followed that up by winning his only title of the year at the Memphis Open.
His form faltered slightly with fourth round defeats at the French Open and Wimbledon, before he rediscovered his touch to win bronze at the Rio Olympics and reached the semi-finals of the U.S. Open, losing to eventual champion Stan Wawrinka.
Nishikori was also a losing finalist at the Miami Open in April and Rogers Cup in July before his season ended with a defeat to Novak Djokovic in the semi-finals of the ATP World Tour Finals in London on Saturday.
The 26-year-old remains hopeful he can climb up to number three in the world rankings, one better than his career-high.
"I'm confident I have the ability to challenge for world number three and four," he added.
"I need to be more cautious while determining which points in the match I have to focus and how I can win break points in a more assured manner."
(Reporting by Debanjan Bose in Bengaluru; Editing by John O'Brien)
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