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.
The tourists limped to the close on 117-4, still 375 short of their target with a day’s play remaining.
Elgar had been fortunate to survive on nine, dropped by Keaton Jennings at third slip off Jimmy Anderson, and went on to strike seven fours in his 77-ball half-century.
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.
England’s new captain eventually called time when Jonny Bairstow was dismissed before tea after a breezy 63 which lifted England, who had resumed on 74-1, to 313-8.
After Jennings departed early for a painstaking 48, debutant Tom Westley pushed on to a maiden half-century, an innings that should ensure an extended run at England’s problem number three spot.
If South Africa fail to survive on Monday, it will complete a one-sided sequence in each of the tests. England won by 211 runs at Lord’s before South Africa hit back with an even more emphatic 340-run victory at Trent Bridge. The final test starts in Manchester on Friday, Aug. 4.