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 industrialised nation for more than a decade.
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.
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.
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.
President Cyril Ramaphosa has been trying to reform Eskom to make it more efficient since becoming head of state in 2018 but his administration has made slow progress, fuelling public frustration.
His government has said it will take on part of the utility's 400 billion rand ($23.05 billion) debt, but it is yet to commit to a specific amount or timeframe.
Last month Eskom warned maintenance work would further reduce power availability and exacerbate power cuts over the next year.
($1 = 17.3511 rand)
(Reporting by Bhargav Acharya and Alexander Winning; Editing by James Macharia Chege, Kirsten Donovan)
