Naomi Osaka's opening match in the Olympic tennis tournament has been pushed back from Saturday to Sunday. Organizers did not immediately provide a reason for the switch. They said only that the move came from the tournament referee.

Osaka was originally scheduled to play 52nd-ranked Zheng Saisai of China in the very first contest of the Games on center court Saturday morning.

One reason for the move could be that Osaka might have a role in the opening ceremony Friday night. That wouldn't leave her much time to rest before a Saturday morning match.

Osaka is returning to competition for the first time in nearly two months after she withdrew from the French Open following the first round to take a mental health break.

She is one of Japan's top athletes.