"I had doubts about my decision (to play at the U.S. Open) but I am going to take a chance," the Kyodo news agency quoted Nishikori as telling reporters in New York on Friday.
"However, to be honest I have some concerns about my physical fitness."
The injury-prone 24-year-old, the first Japanese to break into the top 10 of the men's singles rankings, lost to Briton Daniel Evans in the first round of the U.S. Open last year.
This year he will play 180th-ranked American Wayne Odesnik in the opening round.
"I haven't had match practice and I don't know if I will be able to keep up when it comes to quick movement," Nishikori added. "If the pain comes back on the day of the game I don't know if I will play.
"I don't know if I will be able to get through the first round."
Nishikori's best performance at Flushing Meadows came in 2008 when he reached the fourth round as an 18-year-old.
Despite doubts over Nishikori, Japan will be well represented in the men's draw after Taro Daniel, Yoshihito Nishioka and Tatsuma Ito came through their third-round qualifiers on Friday.
The 21-year-old American-born Daniel and 18-year-old Nishioka, ranked 246 in the world, qualified for the main draw of a grand slam for the first time.
(Reporting by Sudipto Ganguly in Mumbai; Editing by John O'Brien)
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