Japan's top-ranked men's tennis player Kei Nishikori added his voice to concerns about staging the Olympic Games in Tokyo in July, as the country battles a surge in Covid-19 cases.

Naomi Osaka, Japan's No.1 women's tennis player, raised similar concerns on Sunday, saying that the risks of holding the Games amid the pandemic should continue to be carefully discussed.

"This is not like 100 people like these tournaments," Nishikori said at the Italian Open in Rome on Monday.

"It's 10,000 people in a Village, playing a tournament," he added, referring to the Olympic Village where athletes will stay.