Nishikori, who started his professional career in 2007, won 12 ATP Tour titles and has been playing on the Challenger Tour tour this year.
He last competed on the main ATP Tour at the Cincinnati Open in August 2025.
One of Nishikori’s most notable wins was beating 24-time Grand Slam champion Novak Djokovic in the semi-finals of the 2014 US Open.
“Reaching the ATP Tour, playing at the highest level of competition and maintaining a presence in the top 10 is something I am extremely proud of,” said Nishikori.
“Whether in victory or defeat, the special atmosphere I felt in packed arenas is irreplaceable.
“There were also times when I was overwhelmed by frustration and anxiety due to repeated injuries that prevented me from playing as I wanted.
“Even so, my love for tennis and my belief that I could become a stronger player always brought me back to the court.
“I feel that all of these experiences have enriched and shaped my life. I am deeply grateful [to] my family and to everyone who has supported me at all times.”