Osaka falls out of top 10 for first time since 2018
The Japanese star, a former world number one, has not played since exiting the US Open in the third round last month.
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.
WTA top 20 on October 4:
1. Ashleigh Barty (AUS) 9,077 pts
2. Aryna Sabalenka (BLR) 7,115
3. Karolina Pliskova (CZE) 5,285
4. Iga Swiatek (POL) 4,756
5. Barbora Krejcikova (CZE) 4,668
6. Garbine Muguruza (ESP) 4,595 (+3)
7. Elina Svitolina (UKR) 4,376 (-1)
8. Sofia Kenin (USA) 4,190
9. Maria Sakkari (GRE) 4,055 (+1)
10. Belinda Bencic (SUI) 3,835 (+2)