Kei Nishikori remains positive for coronavirus ©Getty Images

Japanese tennis star Kei Nishikori is a doubt for the US Open after confirming he has tested positive for coronavirus for a second time.

Nishikori initially tested positive last Sunday (August 16), which came just 24 hours before he was due to leave his base in Florida for New York.

He was forced to withdraw from the Cincinnati Masters - also known as the Western and Southern Open - as a result.

The tournament is serving as the US Open warm-up event and has been moved from Cincinnati to New York City due to the pandemic.

Nishikori said he remain in isolation in Florida and will take another test early next week.

"Just a little update, I just took another Covid test and am still positive," Nishikori wrote on his official app.

"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."

The 2014 US Open runner-up and Olympic bronze medallist will not be able to participate at the Grand Slam event unless he recovers from coronavirus.

Players competing at the US Open have to enter a "bio-secure bubble", including mandatory tests before being allowed into the Flushing Meadows site.

The US Open has been hit by a series of high-profile withdrawals, including defending champions Rafael Nadal and Bianca Andreescu.

Wimbledon champion and world number two Simona Halep confirmed she was skipping the tournament at Flushing Meadows earlier this week.

It means six of the top 10 women's players will be absent from the event.

Germany’s Angelique Kerber, the 2016 US Open champion, confirmed on Instagram today that she will participate.

Kerber has not played since the Australian Open in January.

"I can’t wait to get back on court and compete,” Kerber wrote.

"This is why me and my team have decided to take on the challenge and fly to NYC in a few days.

"Thanks to the WTA and USTA for their huge effort to make this a safe event.”

Serena Williams, a 23-time Grand Slam champion, is the only one of last year's women's semi-finalists to enter this year's competition.

Williams and men's world number one Novak Djokovic are among those to have confirmed their participation.

The US Open, traditionally the last Grand Slam of the year, will be played without spectators and with safety measures in place, some of which have been criticised by the likes of Djokovic.