Osaka pulls out of Stuttgart WTA
The tournament in south-west Germany from April 17-25 is set to go ahead behind closed doors due to the Covid-19 pandemic.
It is one of the first clay-court events in Europe in the build up to the French Open at Roland Garros, which is due to start on May 23.
Osaka won the first Grand Slam title of 2021 last month when she beat Jennifer Brady of the USA in the Australian Open final.
As well as defending champion Petra Kvitova, top-ten stars Simona Halep, Sofia Kenin, Elina Svitolina, Aryna Sabalenka and 2018 Stuttgart winner Karolina Pliskova are also expected to be feature.