Barty has held the number one spot since the beginning of September having also occupied it in July and August.
Pliskova of the Czech Republic has not reached number one since summer 2017.
There were no changes in the top 10.
WTA rankings as of September 23:
1. Ashleigh Barty (AUS) 6,446 pts
2. Karolina Pliskova (CZE) 6,125
3. Elina Svitolina (UKR) 5,160
4. Naomi Osaka (JPN) 5,011
5. Bianca Andreescu (CAN) 4,835
6. Simona Halep (ROU) 4,803
7. Petra Kvitova (CZE) 4,326
8. Kiki Bertens (NED) 4,225
9. Serena Williams (USA) 3,935
10. Belinda Bencic (SUI) 3,738
11. Johanna Konta (GBR) 3,073
12. Angelique Kerber (GER) 2,835 (+3)
13. Madison Keys (USA) 2,827 (+3)
14. Aryna Sabalenka (BLR) 2,785 (-1)
15. Sloane Stephens (USA) 2,769 (-1)
16. Qiang Wang (CHN) 2,668 (-4)
17. Sofia Kenin (USA) 2,630 (+3)
18. Caroline Wozniacki (DEN) 2,597 (-1)
19. Anastasija Sevastova (LAT) 2,517 (-1)
20. Anett Kontaveit (EST) 2,500 (-1)
