Japan's former US Open finalist Kei Nishikori has withdrawn from next week's Grand Slam tournament despite recovering from coronavirus.

"After such a long break I feel returning in a best-of-five sets setting is not smart," said the 2014 runner-up at Flushing Meadows.

Meanwhile, injury-hit 2009 champion Juan Martin del Potro, 31, has had a third knee operation in two years.

The former world number three, who also reached the 2018 final at Flushing Meadows, has not played since fracturing his right kneecap for a second time at Queen's Club in June 2019.

The Argentine suffered the initial fracture in October 2018 in Shanghai, making a comeback a few months later before slipping on the grass during a second-round match with Canada's Denis Shapovalov at the Fever Tree Championships.

Del Potro had been back on court in Buenos Aires earlier this year, but decided to have more surgery when the pain persisted.