Got an e-mail from TK:
"Dear Passengers,
For flights to our country which have STD (schedule time of departure) 30 December 2020 00:00 or later with the dates and times of the countries where the flights are operated , 6 years and older passengers who will enter Turkey must submit negative PCR test documents taken 72 hours before the schedule time of departure of the flight (maximum 72 hours between the sample delivery time and the schedule time of departure of the flight). It must be nucleic acid test. Transit passengers who will not enter Turkey only should pay attention to the destination country regulations. This implementation is valid until March 1, 2021.
Moreover, passengers (6 years and older) traveling from UK, Denmark or South Africa with the flights that have STD (schedule time of departure) 30 December 2020 00:00 or later with the dates and times of the countries where the flights are operated or passengers have been in these 3 countries in the last 10 days and will enter Turkey are required to submit negative PCR test documents taken 72 hours before the schedule time of departure of the flight (maximum 72 hours between the sample delivery time and the schedule time of departure of the flight). These passengers will be quarantined at their residences (if any) or in different places and they will be subjected to a PCR test 7 days later. If the test result will be negative, quarantine will be removed.
We wish you a good flight."
So... don't try to enter turkey after being in SA or you'll need to quarantine for a week.