South Africa were cruising at 220 for two just after tea when Cummins returned to the attack for a magnificent spell of bowling during which he picked up four wickets for 12 runs in eight overs, including the key scalp of AB de Villiers (64), to trigger a South African collapse.
The home side ended the day on 266 for eight after dominating the first two sessions.
"I felt like I was pretty ordinary in my first 10 overs and I was worried I was going to be a passenger out there," Cummins told reporters. "But the ball started swinging and we felt if we could get one wicket, two or three would follow.
"I bowled a half-volley that AB hit to mid-off and after that I felt I had my rhythm. Wickets always make the legs feel a little bit fresher.
"The AB wicket was the big one, once he was out a new batsman starting his innings against a reversing ball … it’s pretty hard. It got us right into the game."
Cummins, who ended the day with 4-64 in 21 overs, believes the touring side are now on top in the game as they seek to take the lead in the four-match series which is poised at 1-1.
"It was a brilliant last session, to have them eight down and only 260-odd … it all depends on us getting those last two wickets in the morning but at this stage we are really happy."
Opener Dean Elgar struck an excellent unbeaten 121 for South Africa and he will resume in the morning with Kagiso Rabada on six.
"That’s the nature of test cricket, for four hours a team can be dominant and for two hours the other team can bounce back and also be dominant and put the test in the balance," Elgar said.
"The wicket is by no means flat, it might have looked like it when AB was batting, but he does that. They (Australia) identified areas they could exploit. The older ball, which was reversing, makes batting trickier."
Elgar revealed that the chatter from the Australian fielders was not unusual in an acrimonious series in which five players have been sanctioned by the International Cricket Council.
"It was stock-standard, just noise, which we know is going to happen," Elgar said.
"It was quite humorous actually, I enjoyed it today. You are always going to have that competitive edge, there are a lot of guys with a big egos out there."
First Published: March 23, 2018, 7:50 AM IST