England ripped apart South Africa's top order in Sunday's final session at the Oval to bear down on a victory that would put them 2-1 up with one to play in the four-test series.
Two wickets from successive balls in Ben Stokes' third over removed Quinton de Kock (5) and Faf du Plessis (0) and ended faint South Africa hopes of mounting a rearguard action in pursuit of their nominal target of 492.
Although Stokes failed to record what would have been the first-ever test hat-trick at the Oval, the damage was done. Du Plessis' wicket, lbw without playing a shot to a first-ball inswinger, was a particular blow, with South Africa needing their captain to survive at least into Monday.
The tourists limped to the close on 117-4, still 375 short of their target with a day's play remaining.
The battling Dean Elgar was unbeaten on 72 as England turned to Moeen Ali to try to secure another breakthrough.
No one else provided much resistance. Before Stokes struck, Stuart Broad removed opener Heino Kuhn for 11 and Toby Roland-Jones dismissed Hashim Amla for five.
England have a further 98 overs available on Monday to complete the victory and vindicate Root's decision to delay his declaration to apply maximum pressure to South Africa.
SCORES IN BRIEF
ENGLAND: First innings 353
SOUTH AFRICA: First innings 175
ENGLAND: Second innings 313 for 8 decl (Jennings 48, Westley 59, Root 50, Stokes 31, Bairstow 63, Roland-Jones 23 not out; Morris 2-70, Maharaj 3-50)
South Africa: Second innings 117 for 4 (Elgar not out 72, Bavuma not out 16; Roland-Jones 1-20; Stokes 2-29)