Had it not been for the 19-year-old's unbeaten 68 (136 balls, 8x4), the Gestetner Diamond Eagles would have failed to maintain the advantage they gained when they bowled out the Nashua Dolphins for 132 inside 40 overs after losing the toss at the De Beers Diamond Oval.
Rossouw already has three centuries behind his name this summer and the only thing that is likely to stop him adding a fourth is the fact that he may run out of partners as happened in the previous round.
One probably has to go back to the emergence of Peter Kirsten in the early 1970s to find the last time a No. 3 batsman showed such promise at such a young age.
The Diamond Eagles finished the day on 170/7 for an overall lead of 38 runs.
Alviro Petersen of the bizhub Highveld Lions provided the only century of the day as he and Dane Vilas shared an unbroken 205-run fifth wicket partnership in their match against the Nashua Titans at the Liberty Life Wanderers Stadium.
Petersen scored his 14th first-class century to finish the day on 119 (157 balls, 17x4, 3x6) while Vilas contributed 83 (162 balls, 8x4, 1x6) in a partnership that came at the rate of more than four to the over and took the Lions to 317/4 by the close of play.
The Chevrolet Warriors failed to build on several good starts by their batsmen at Buffalo Park to be bowled out for 237 by the Nashua Cape Cobras for whom Claude Henderson took 7/64.
Arno and Davy Jacobs and Zander de Bruyn all reached the 50s but nobody went on to the big score that was needed.
The Cobras made a hesitant start to be two wickets down for one run with Andrew Puttick and Stiaan van Zyl facing three balls between them before being bowled by Andrew Birch.
Alistair Gray and Justin Ontong managed to steady the ship, taking the Cobras to 27/2 at the close.
