Naomi Osaka says rising Covid-19 levels in Tokyo are a "big cause for concern" and she is "not really sure" the Olympics should go ahead this summer.

Japanese world number two Osaka would be making her Olympic debut in Tokyo.

"I'm an athlete, and of course my immediate thought is that I want to play in the Olympics," she said.

"But as a human, I would say we're in a pandemic, and if people aren't healthy, and if they're not feeling safe, then it's definitely a really big cause for concern."

When asked by BBC Sport if it would be appropriate to stage the Games during a pandemic, the 23-year-old said: "To be honest, I'm not really sure."

The Olympics will take place without international fans, while a decision on whether local fans can attend the Games will be made in June.

"I've never played an Olympic event, so it's not like I would have anything to compare it to," four-time Grand Slam champion Osaka said.

"I would feel, of course, definitely a bit sad, but at the end of the day, it's an honour to play in the Olympics in the first place and if that's what keeps people healthy, then I'm up for [playing behind closed doors]."

Williams undecided over Tokyo

Meanwhile, 23-time Grand Slam champion Serena Williams has raised doubts over her participation in the Tokyo Olympics.

Williams, who has won one singles and three doubles Olympic gold medals, is hesitant to spend time away from her three-year-old daughter, Olympia.

"I haven't spent 24 hours without her, so that kind of answers the question itself," said the American, 39.

"I haven't really thought much about Tokyo, because it was supposed to be last year and now it's this year, and then there is this pandemic and there is so much to think about.

"Then there are the Grand Slams. It's just a lot. So I have really been taking it one day at a time to a fault, and I definitely need to figure out my next moves."

Williams, ranked eighth in the world, is scheduled to play at this week's Italian Open, her first tournament since losing to eventual champion Osaka in the semi-finals of the Australian Open in February.