Heinrich Klaasen and Kyle Verreynne both hit double hundreds in the 4-Day Series on Friday.
Klaasen smashed 292 off 240 balls (39 fours and nine sixes) as the Northerns Titans were bowled out for 648 at Centurion.
The Free State Knights were 190-5 at stumps, with Gihahn Cloete unbeaten on 46. Lizaad Williams took 3-38 and Junior Dala – who made 56* with the bat – 2-10.
At Newlands, Verreynne scored 201* off 221 balls (21 fours and six sixes) as Western Province declared their first innings closed on 557-5.
The Boland Rocks were then bowled out for 202, with WP 12-1 in their second innings at stumps and leading by 343 runs.
In Port Elizabeth, the EP Warriors are 158-4 in their second innings after bowling out the KZN Dolphins for 285 (Jason Smith 45, Andile Phehlukwayo 68, Eathan Bosch 51).
The hosts, who have Matthew Breetzke (43*) and Diego Rosier (16*) at the crease, lead by 39 runs.
In Potchefstroom, Bjorn Fortuin took 4-38 for the Central Gauteng Lions to leave the North West Dragons in disarray on 143-9, still trailing by 219 runs.