The former world No.4 four withdrew from the Western and Southern Open in New York after contracting COVID-19.
Nishikori may be forced to miss the second gram slam of the year, which starts at Flushing Meadows on 1 September (AEST), after discovering he still has the virus.
The 2014 US Open runner-up posted on social media: "Just a little update, I just took another COVID test and am still positive.
"I have very minimal symptoms and am staying in full isolation in Florida.
"The next test will be early next week at which time we will update you with more information."
Nishikori will need to return two negative tests in order to enter the bubble in New York.