Naomi Osaka dropped out of the world's top 10 on Monday for the first time since winning the 2018 US Open title as she continues to take a break from tennis.
The Japanese star, a former world number one, has not played since exiting the US Open in the third round last month.
Osaka has fallen to 12th in the WTA rankings, her lowest mark since before her first of four Grand Slam titles at Flushing Meadows three years ago, when she beat Serena Williams in an infamous final.
Last week, the 23-year-old said that her return would be "probably soon" and that she "kind of" has "that itch again".
Osaka had previously pulled out of the French Open and skipped Wimbledon this year to protect her mental health.
Two-time Grand Slam champion Garbine Muguruza jumped to sixth in the rankings after winning the title in Chicago last week.
Tunisian Ons Jabeur, who lost to Muguruza in the final, climbed to a career-high ranking of 14th.