The 30-year-old former world number four has been staying at the IMG Academy in Bradenton, Florida, and pulled out of the upcoming Western & Southern Open, a warm-up tournament for many before the Grand Slam.
“This morning while still in Florida, I got tested for COVID-19 and tested positive,” Nishikori said in a statement. “I will have to pull out of the Cincinnati tournament at this time.
“We were planning to fly to New York tomorrow but will obviously now stay in Florida.”
Nishikori added that he had “very little symptoms” and would go into self-isolation.
A number of players have withdrawn from this year’s U.S. Open over fears of contracting the coronavirus that has infected more than 5.3 million people in the United States and killed nearly 170,000, according to a Reuters tally.
Defending champion Rafa Nadal and women’s world number one Ash Barty are among those who refused to travel, while reigning women’s champion Bianca Andreescu pulled out, saying the pandemic had compromised her ability to reach her highest level of form.