The experienced lock missed last weekend's 17-16 victory over Wales because of an Achilles problem, but he will start in Wellington alongside second-row partner Danie Rossouw.
The Springboks, though, will be without lock Victor Matfield, wing Bryan Habana, centre Jean de Villiers and former Bath back Butch James, who are all nursing injuries.
"I will still feel a little bit of stiffness," said Botha.
"But it's like that old farm vehicle in the morning - it smokes a bit but, when you drive it, after an hour, it's hot and it can go.
"I definitely know I am ready for this weekend. I truly believe that when you pull on the green and gold jersey you must give 100%, and that is what I am going to give.
"If my heel is not going to make it, it's not going to make it. If it lasts for 80 minutes, it lasts for 80 minutes."
In the absence of De Villiers, Frans Steyn moves from full-back into midfield alongside Jaque Fourie, with 20-year-old Pat Lambie handed the number 15 shirt for his first World Cup start.
Despite South Africa's injury troubles, Fiji still expect a trademark ferocious challenge from the Springboks.
"If you look at their 30-man squad, there is strength and depth in a lot of positions," said Fiji skipper Deacon Manu.
"I know they are missing some of their main players who they've gone to in the past, but the guys who have stepped up thoroughly deserve to be there.
"They are great players in their own right so, in terms of preparing for the game, it's not going to make too much difference for us.
"We need to make sure we are firing on all cylinders if we want to challenge them.
"We have got to make sure as a pack that we stand up and we match them physically, mentally and tactically in all the areas that will let us compete with them on the scoreboard."
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