Harmer contributed four wickets as South Africa eventually bowled the visitors out for 429 on the third day, a hundred more runs than the hosts’ first innings total in Cape Town.
He says the wicket at Newlands has changed quite a bit and has become more sub-continent-like, offering more turn for the likes of him and Australia’s Nathan Lyon.
"The wicket has changed quite dramatically, so I think spin will come into the game a lot more than it has during the first two and a half days because it was quite evident this evening how the ball was turning after hitting the foot marks."
Harmer says the aim was to contain the Australians as much as possible coming into Day 3, and getting wickets was the only to do that.
Having covered the deficit in their second innings, the Proteas have established a lead of 40 for the loss of four wickets.
Harmer stressed how key the first hour of the fourth day will be, with Matthew Breetzke and Ryan Rickelton at the crease for them.
"The most important part of this game for us now is that first hour in the morning because we can't get caught up in what would be a good total or a defendable score, because a defendable score is not achievable if that first hour doesn't go well."
The 37-year-old believes while there’s been some concerns regarding the spectator attendance at the three matches, this has been one of the most entertaining and closely contested series between the two great rivals in recent memory.