Former French Open winner Iga Swiatek was rewarded for her run to the semi-finals in Ostrava last week with a two-place rise to a career-high number four in the WTA rankings released on Monday.
The 20-year-old Pole, victorious last year at Roland Garros, has struggled on the Grand Slam circuit this year but has won two tournaments, including the WTA 1000 in Rome in the spring.
The Greek Maria Sakkari, who beat Swiatek in the semi-final in Ostrava before losing in the final, also climbs two places to enter the top 10 for the first time.
Elina Svitolina, the Ukrainian top seed at this week's WTA event at Chicago where four-time major winner Kim Clijsters returns to action, drops two places to number six.
Former world number one Serena Williams, who turned 40 on Sunday, remains at number 40.
WTA rankings as of September 27:
1. Ashleigh Barty (AUS) 9726 pts
2. Aryna Sabalenka (BLR) 7005
3. Karolina Pliskova (CZE) 5265
4. Iga Swiatek (POL) 4756 (+2)
5. Barbora Krejcikova (CZE) 4668