Rookie Yorkshire batter Brook has been drafted into the hosts' team for a tantalising finale at the Kia Oval in place of the injured Jonny Bairstow.
The Proteas have already been on the receiving end of the 23-year-old's talents after he last month smashed 140 during a crushing win for the England Lions in a tour match in Canterbury.
Elgar downplayed the importance of that display and insists Brook faces a far different proposition over the coming days.
"You can bat like that in a four-day game or a County Championship game but this is Test cricket," the 35-year-old said.
"There a lot of things that put a stop to you as a player, whether it's the crowd, the cameras, whatever the case is.
"Brooky, I've played quite a few games against him, being around the county circuit. He's a good player, no doubt.
"He got a few runs against us but, again, this is Test cricket and we've got our fast bowlers now who didn't bowl against him in Kent, so I'm not reading too much into that.
"This is Test cricket, it will humble you as a player and a person. Hopefully he doesn't do what he did against us in Canterbury.
"I can't blow too much smoke for opposition, I'm not there to do that. But I just know the environment and this is the big league now."
South Africa are seeking a reaction after their Old Trafford thrashing inside three days left the series level at 1-1.
Ryan Rickelton is likely to replace the injured Rassie Van Der Dussen, with Elgar suggesting there may be a "few extra changes".
While the weather forecast suggests rain could interrupt play, opener Elgar is confident there will be a winner and has no interest in settling for a tied series.
"I think it's the biggest Test so far in my captaincy period," he said.
"I've never experienced a Test (series) win against England from playing for 10 years, which would be an unreal feeling personally but massive for the younger guys in our changing room.
"It's not make or break but it's kind of big going forward for us from a confidence point of view.
"There's definitely going to be a winner in this Test, no doubt. The styles of cricket that we've been playing, there's definitely going to be a result.
"It's pretty much like a World Cup final for us, that's the way I'm viewing it."