Stage 6 power cuts require up to 6,000 megawatts to be shed from the national grid and mean at least six hours a day without power for most South Africans.
Eskom said the Stage 6 cuts would last until 0500 local time (0300 GMT) on Friday, after which they would be lowered to Stage 5 until 0500 on Saturday.
They were implemented for the first time ever in December 2019, and again earlier this year.
"This is due to a high number of breakdowns since midnight, as well as the requirement to strictly preserve the remaining emergency generation reserves," Eskom said in a statement.
The utility, whose ageing fleet of coal-fired power stations is prone to faults, said a flurry of breakdowns and delays returning other units to service had caused the escalation in outages.
Eskom has struggled to meet electricity demand in Africa's most industrialized nation for more than a decade.
Eskom has also been a centerpiece of the "state capture" allegations lodged against former President Jacob Zuma, which assert the utility was effectively looted by persons connected to his administration.