NEW YORK (AFP) - Japanese tennis star Kei Nishikori said on Friday (Aug 21) he is still positive for Covid-19 and will remain in isolation into early next week as the start of the US Open looms.

The 2014 US Open runner-up tested positive in Florida last Sunday and withdrew from the ATP tuneup that starts Saturday in the same Covid-19 bubble atmosphere where the New York hardcourt Grand Slam begins Aug 31.

Should he continue to test positive into next week, Nishikori would be in jeopardy of missing the start of the US Open.

"I just took another Covid test and am still positive," Nishikori said in a social media post on Friday.

"I have very minimal symptoms and am staying in full isolation. Next test will be early next week at which time we will update you."

World number 31 Nishikori has produced his best Grand Slam efforts at Flushing Meadows, including his loss to Marin Cilic in the 2014 final and semi-final runs in 2016 and 2018.

Nishikori, whose most recent victory came in 2019 at Brisbane, has won six of 12 career ATP titles on US soil.

The potential loss of Nishikori from the first Grand Slam event since the global Covid-19 pandemic shut down the tour season is another blow for the US Open, which will already be without defending champion Rafael Nadal, injured star Roger Federer, reigning women's champion Bianca Andreescu and top-ranked Ashleigh Barty.

Among those who are planning to compete in the Flushing Meadows fortnight are top-ranked Novak Djokovic, Britain's Andy Murray and 23-time Grand Slam winner Serena Williams.