Joe Root's partnership with Ben Stokes was worth 111 runs on day two
The Yorkshireman's ninth Test ton rescued England from 91-4 to 238-5 by the close, just 75 runs behind South Africa's first-innings 313.
It was the first time Root had reached three figures in Tests since last summer's surprise Ashes triumph against Australia, when he posted centuries in the pivotal victories at Cardiff and Trent Bridge.
But asked if this latest knock, which he will restart today on 106, could be his finest for England, Root said: "Yes, but we're going to have to do a lot of hard work to get a positive result and the rest of this innings is going to be crucial.
"We'll have to build some strong partnerships if we want to get a lead. If we can do that we can put South Africa under pressure."
England, 1-0 up, know a win here will seal the series for them. Root, though, was just glad to reach a hundred after failing to convert 10 of his 13 half-centuries in 2015.
"There was quite a lot of relief when I managed to get past three figures," he said. "Unfortunately I've not managed to do that recently. So now's the time to push on and make a big one."
Root's 111-run fifth-wicket stand with Ben Stokes helped England recover from their wobbly start.
And Root paid tribute to the Durham all-rounder after he followed up last week's breathless double hundred in Cape Town with a lightening-quick 58 here.
"He takes pressure off you at the other end when he comes in and plays that aggressively," said Root. "Hopefully he can continue the great form he's in on the rest of this trip.
"When you see someone scoring at the other end like that and putting the bowlers under pressure it brings that out in your own game as well. It's nice to have that.
"He scored at over a run-a-ball and in the match scenario that was really important for us. The quicker we can score the better chance we have of getting a positive result in this game."
Asked what sort of lead England would need to win this match, Root added: "I'd like 250 but whatever we can get really. Any form of a lead would be invaluable."