After her victory at the Australian Open over the weekend, 21-year-old tennis star Naomi Osaka is being feted in Japan, which finally has its first-ever tennis player to be ranked No. 1. In less than nine months time, however, she faces a difficult choice that will determine whether she can ever play for Japan again.

Osaka, whose mother is Japanese and father is Haitian, was born in Japan but spent most of her life in the US, and holds both Japanese and American citizenship. By Japanese law, however, anyone with dual citizenship (paywall) must choose whether they want to retain or give up their Japanese passport when they turn 22 as Japan does not allow its citizens to hold other nationalities.

Japan’s strict law on citizenship has long been debated and criticized, particularly as more and more people in Japan are either born with one foreign parent or have spent enough time overseas to gain citizenship in their country of residence. About 27% of the world’s countries automatically revoke the citizenship of those who get a second one, including Norway.

Osaka’s father said that (paywall) he and his wife decided when Osaka was young that she would represent Japan rather than the US, where the family has lived since she was three, because they have “always felt Japanese.” Japan’s tennis association supported Osaka from a young age, while its US counterpart did not vigorously try to poach her until 2016 when she played in the Australian Open. Osaka’s father decided that his daughter would continue to represent Japan because of Japan’s long-standing support of Naomi, Japanese media reports (link in Japanese).

But Osaka’s recent successful run—starting with her victory at the US Open in September—has reinvigorated the discussion to a fever pitch. Michio Ushioda, a veteran journalist at Japan’s Mainichi newspaper, said in a tweet (link in Japanese) that as the date of Osaka’s choice looms, he believes that it’s likely that she’ll choose to give up her Japanese citizenship for her American one. The disappointment in Japan will be so great, he added, that it could even lead to the downfall of the Japanese government.

Ushioda’s comments may be an exaggeration, but they illustrate the added urgency of the discussion over Japan’s citizenship laws as the country prepares to host the Olympics next year. The country will no doubt want Osaka to choose her Japanese passport over American.

Osaka’s predicament is one that is shared by thousands of people who have parents of different nationalities or have the citizenship of other countries by birth. In practice, however, the justice ministry has never revoked the Japanese citizenship of any dual nationals by birth, according to a detailed report by the Japan Times (paywall) last year. One high-profile individual who was ensnared by the law, however, was opposition lawmaker Renho, who was born to a Taiwanese father and Japanese mother in Japan, and was forced to publicly prove in 2017 that she had given up her Taiwanese nationality.

Some Japanese nationals who became naturalized citizens of other countries, however, have had their Japanese nationality canceled. A group of Japanese citizens residing in Switzerland, France, and other European countries, for example, last year sued the government over being forced to relinquish their Japanese citizenship after they acquired foreign citizenship in order to continue living and working in those countries.

Whatever Osaka does when her birthday arrives on Oct. 16, the current debate is clearly about much more than sports.

Hers is a situation that will become increasingly commonplace in a Japan that is fast shedding its racial homogeneity, particularly as the country opens its doors wider to foreign workers starting this year to tackle its labor shortage. Osaka’s success has also advanced the discussion in Japan over what being Japanese means (paywall), in a society where many mixed-race Japanese still face discrimination. Officially allowing dual citizenship is a step toward acknowledging a more diverse and inclusive Japan.