Photo/Illutration Kei Nishikori (Asahi Shimbun file photo)

Kei Nishikori's hopes of playing at the U.S. Open have faded further after the Japanese former world number four revealed he had tested positive for COVID-19 for a second time.

The 30-year-old, a finalist at Flushing Meadows in 2014, withdrew from the Western & Southern Open hardcourt warm-up after his first positive test last week.

"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," Nishikori posted on social media late on Friday.

"The next test will be early next week at which time we will update you with more information."

Nishikori, who has been staying at the IMG Academy in Florida, has enjoyed his greatest Grand Slam success at the U.S. Open, reaching the semi-finals in 2016 and 2018 in addition to his loss to Marin Cilic in the 2014 title-decider.

The U.S. Open starts on Aug. 31 but a string of top players have withdrawn over fears of contracting the novel coronavirus, which has infected more than 5.6 million people and killed 175,397 in the United States. Defending champions Rafa Nadal and Bianca Andreescu as well as women's world number one Ash Barty are among those who have decided not to travel to New York.