England must turn late pressure on day four into victory on day five as they look to take an early lead in the Test series, says Steven Finn.
The fast bowler struck three times before the close of play at Kingsmead to leave South Africa 136-4, trailing by 280 runs.
After allowing South Africa quick early runs in their chase of 416, England struck back just before tea as Ben Stokes removed Stiaan van Zyl (33) before Finn came to the party.
A more disciplined display saw the runs dry up in the final session, with Finn (3-27) benefiting from the pressure building on the hosts to remove Hashim Amla (12), Dean Elgar (40) and Faf du Plessis (9).
"We built pressure really nicely through that last period of the day after tea and so to get those four wickets before the close of play was really satisfying," he told SuperSport.
"We knew they were going to come at us hard and go for the runs, and they played really well, and we chased wickets slightly in those first few overs before tea.
"It's tricky to know when to attack and when not to with the ball, but we have got some really experienced players within the team which helps to judge that
"We have to come back and be patient, we can't chase it because they could get away and we would lose the pressure we worked hard to build up."
AB de Villiers (37 not out) remains unbeaten going into day five after Jonny Bairstow missed a late stumping attempt off Moeen Ali (0-36), and Finn knows his wicket could prove crucial.
He added: "Ideally we would have liked to have seen the back of him, but collectively we bowled really well to him.
"We know he's a world-class player, he will be a big wicket for us if we can get him."