The third and deciding Dafabet Test between South Africa and Australia - and hence the three match series - is delicately poised at the close of day three with the hosts leading by just 40 with six wickets in the bank at YesPlay Newlands in Cape Town on Thursday.
"It's clichéd and simplistic but the first hour is going to be vital tomorrow morning," stated Harmer.
Matthew Breetzke (19*) and captain Temba Bavuma (35) fought valiantly but five minutes before the close, Nathan Lyon (14-3-31-1) broke through Bavuma's defences to alter the balance slightly.
"I think everyone would have wanted us to be three down at stumps. I don't think anyone is going to be as shattered as Temba for getting out," stated Harmer. "We have two capable batters at the crease and a lot of batting to come. There will be casualties along the way but you hope some small things will start to go your way!" added the Proteas spinner.
"Spin is going to come into it a lot more than it has in the first two and a half days. It was quite evident to see tonight how the ball was turning out of the footmarks. I'm not sure if the middle of the pitch is going to start going up and down for the seamers but I think spin has a massive role to play," said Harmer at the post-day press conference.
Ryan Rickelton (8*) walked out to at the most difficult time but along with Breetzke, took South Africa to the close of play to prevent Australia from gaining any further foothold in the match after Aiden Markram (1) and David Bedingham (11) were dismissed prior to tea and Marques Ackerman (62) departed to leave South Africa 140 for four at the close.
"We can't get caught up in what we want or what would be nice or what is defendable because it is not achievable if that first hour does not go well," said Harmer. "I'll say this in the most respectful manner, those players that feel those shots were not good enough will be the first people to put up their hands. Everyone wants to contribute and win the game for the team," added Harmer.
Australia began the third day in a powerful position, 300 for four and leading by one run, with Josh Inglis (102) and Alex Carey (80) at the crease. The pair had already shared in an unbeaten 145 run stand and looked to push on under sunny Cape skies.
"It felt like real Test match cricket, attritional. The ball was a bit softer, they had a ring field and it was quite tough to score. I wasn't hitting the ball the way I wanted but I knew that if I just hung in there, then it would be easier when the new ball comes on," said Inglis at the close of play.
However, Simon Harmer (37-6-104-4) removed both set batters. First, he had Inglis caught behind by Rickelton but not before registering his second Test ton and ending the 5th-wicket stand of 191. Then he had Carey pushing forward and edging to Markram at first slip to leave Australia 379 for six.
"It's a really good wicket and once I got out there, I knew we had to get some sort of lead and I was disappointed that we didn't get some more this morning. If Alex and I could have added about 50 more, that would have been nice," said Inglis.
Gerald Coetzee (19-4-70-1) and Kagiso Rabada (23-1-98-1) contributed with the wickets of Mitchell Starc (11) and Pat Cummins (0) respectively as Australia went to lunch on day three at 411 for eight, an overall lead of 82.
However, the Proteas needed just 25 minutes to close out the innings as Wiaan Mulder (21-5-63-1) had Lyon (1) caught by Harmer at slip and Marco Jansen (22.1-7-60-3) finished off the innings when he took a sharp catch off his own bowling to dismiss Cooper Connolly (29).
With Australia being dismissed for 429 - losing their last six wickets for just 59 runs - South Africa would have been well-pleased to restrict the overall lead to 100.
"The further it goes into the game, it's only going to get harder and harder. I'm not sure of a total, it depends on how long they bat for. We don't want them getting any more than 200," said Inglis.
"If you look at how the wicket has played from this morning to this evening, there has been quite a dramatic change. Mitchell Starc does a pretty good job of churning it," said Harmer.
However, Starc (9-1-29-2) had Markram driving straight to Travis Head at mid-off and then Bedingham played a loose hook-shot from the bowling of Josh Hazlewood (11-1-37-1), with Lyon taking a good catch at deep backward square-leg.
Ackerman batted superbly, bringing up his second-consecutive half-century from 64 balls before gloving a sharp-short ball from Starc through to Cameron Green at first slip. Ackerman looked to review but ran out of time and was sent packing.
Despite some nervous moments, Breetzke, who now has his highest Test score, and Rickelton battled through to the close to setup an intriguing fourth day after Bavuma had raised 4000 Test runs.
Cummins had won the toss on the first morning and asked South Africa to bat first, and the decision looked to backfire as South Africa posted an opening stand of 158 between Markram (78) and Ackerman (95).
Despite Bedingham (50) and Jansen (52) notching up half centuries and a 41 from Rickelton, Starc(16.3-2-72-6) blew the South African lower order away with the new ball under cloudy skies. Starc's spell of 6-12 in six and a half overs had South Africa losing six wickets for 93 runs.
Lyon (2-72), Connolly (1-41) and Hazlewood (1-59) all contributed to South Africa's downfall.
Having begun the second day with an opening stand of 58 between Head (41) and Matt Renshaw (74), Harmer struck twice before lunch, removing Head and Marnus Labuschagne (0) to leave the visitors on 90 for two at lunch.
South Africa again only tasted success twice in the middle session, as Jansen (18-5-55-2) had Renshaw caught behind and then Green (65) caught in the slips by Harmer.
The evening session belonged solely to Australia as Inglis and Carey added 145 unbeaten runs for the fifth wicket to put their side one run in the lead by the close. Carey had struck four boundaries in his 126 deliveries to head off unbeaten on 51 while Inglis was more aggressive, striking nine fours and two sixes from 111 balls to end the day on 85.