His run to the final in Acapulco, where he was beaten by Spain's David Ferrer, elevated him to a new career-high ranking of four, taking him above Britain's Andy Murray.
Novak Djokovic, Rafa Nadal, Roger Federer and Murray have long been regarded as the "big four" in the men's game.
Nishikori, who became the first Asian male to reach a grand slam singles final at the U.S. Open last year, has equalled the highest ranking for a Japanese player, matching the feat of Kimiko Date-Krumm in the women's list 20 years ago.
"What's important is where you are at the end of the year," the 25-year-old Nishikori told local media.
"I hope I can produce some big results."
He will be in action for Japan later this week in a Davis Cup World Group first-round clash against Canada in Vancouver.
(Reporting by Martyn Herman, editing by Ed Osmond)
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