Johanna Konta’s absence from the list of wildcards for next week’s Kremlin Cup means Muguruza – sixth in the Road to Singapore standings – can no longer be caught.
If fit, Muguruza will contest the Finals for the second season in a row, having reached the semi-finals in 2015.
However, the Spaniard decided to bring an end to her last-eight clash with Viktorija Golubic at 4-4 in the deciding set, having earlier appeared to roll on her ankle.
No update has yet been given on Muguruza’s condition, although the confirmation of her Finals qualification could well have prompted a cautious approach to the injury.
Madison Keys, Carla Suarez Navarro and Dominika Cibulkova are all right in the hunt for one of the remaining two Finals places, and there were wins for all three on Friday.
Keys will meet Golubic in the semi-finals following a 6-3 6-3 triumph over Oceane Dodin, while Suarez Navarro battled to a 6-4 4-6 6-2 victory against Denisa Allertova.
Cibulkova awaits Suarez Navarro in the last four after beating Anastasia Pavlyuchenkova 7-6 (7-3) 6-4.