The Proteas confirmed on Saturday that Rabada had been cleared to play at St George's Park, after overcoming a hamstring tear that ruled him out in Durban.
Rabada has an imposing record against Australia, with his 58 wickets coming at an average of 21.39.
The quick is also known to revel in heated contests, with South Africa needing to triumph in Gqeberha to have any chance of winning the series.
It was at St George's Park during Australia's last visit in 2018 that he collided with Steve Smith in a post-wicket celebration, on his way to 11 wickets for the match.
Asked on Saturday whether Australia would make a point not to poke the bear, Cummins suggested that may be irrelevant.
"I know KG a little bit and it doesn't take much to get himself up for the contest," Cummins said.
"Even if you don't poke him, I'm sure he'll find a way to motivate himself.
"He's obviously a class player, hasn't played much lately, but always seems to rise to the occasion and quite experienced in Test cricket.
"So no doubt he'll be a bit of a challenge."
Rabada claimed 23 wickets in the four-Test 2018 series, after having a ban for his contact on Smith overturned on appeal.
The right-arm paceman also backed it up with nine wickets at Lord's in South Africa's World Test Championship final triumph last year.
Australia showed in Durban they were willing to take a patient approach with their batting, with the first-innings 390 their slowest score above 300 in 14 years.
And Cummins suggested a similar approach may be required against Rabada early, to ensure the danger man does not get on a roll.
"it's something you've got to be aware of," Cummins said.
"One of the pleasing things about our batting last week was that trust that you can get through a spell and you can bat long periods of time.
"It is one of the beauties of Test cricket. You don't have a pressure to score a certain way against someone.
"There are tempos in the game where you want to take them on, and maybe some where you just weather the storm and trust you'll still be around later on."
Australia will hold off until the toss to decide their XI, with rain forecast for parts of the opening two days and the prospect of a delayed toss.
Cummins all but confirmed that Matt Renshaw would remain at No.5 after his 190 and 82 in Durban, meaning Nic Maddinson is again likely to open.
The bigger question will likely be whether Australia goes with it's traditional big four bowlers, or if Scott Boland comes in for Nathan Lyon in a shortened match on a green wicket.
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