It was a tale of two nines for the Pole, who saw his three shot overnight advantage evaporate after making bogeys at the third and fourth holes at Leopard Creek Country Club.
But he caught fire after the turn, following up his first birdie of the day at the tenth with further gains on the 11th and 13th to reach 14 under par and sit one stroke clear of nearest challenger Jayden Schaper.
Home favourite Schaper fired an eagle and three birdies on day three to sign for a flawless 67 and catapult himself into contention.
His fellow South African Christiaan Bezuidenhout was alone in third place two shots further back on 11 under after finishing with five birdies in his last eight holes.
European Challenge Tour graduate Meronk had made history on Friday, becoming the first player from Poland to hold the outright lead at the end of a round on the European Tour as he finished the second day three clear at the top.
But a slow start to his third round, coupled with an early charge from his playing partner – last week’s Joburg Open winner Joachim B. Hansen – saw him slip off the summit.
Hansen opened his round with back to back birdies, and with Meronk only able to make pars at both holes, the gap was reduced to one shot.
Meronk’s approach into the third pitched extremely close to the hole but carried on going and finished up just off the green.
He would go on to make his first bogey of the day after missing his par putt, handing Hansen a share of the lead.
And when Meronk made it back to back bogeys with another dropped shot on the fourth, he fell out of the lead altogether.
Hansen led by a single shot at the turn after cancelling out his birdie at the sixth with a bogey on the eighth, while Meronk parred his way to the tenth tee.
Meronk opened his back nine with a birdie at the tenth, slotting in from around ten feet to join Hansen on 12 under.
And when the Pole got another birdie at the 11th and Hansen ran up a quadruple bogey at the same hole after getting into tree trouble, Meronk found himself three clear once again.
Meronk had a spring in his step and came agonisingly close to making it three birdies in a row from the 12th after sending his tee shot to eight feet, but his putt stayed above ground.
He made amends on the 13th, tapping in from a similar distance to get to 14 under.
With 19-year-old Schaper following up an eagle at the 15th with back to back birdies at the 17th and 18th to set the target at 13 under, Meronk was suddenly under a bit of pressure when his tee shot at the 16th landed in a bunker.
But he made a good up and down to save par before finishing his third round with two straight pars to stay in front.
Meronk relished battling it out at the top of the leaderboard and is looking forward to Sunday’s final round.
He said: “I actually enjoyed it very much, even at the beginning.
“It was a lot of fun playing with J.B.
“He got off to a good start so I was trying to catch him.
“It was a nice experience and I’m really looking forward to tomorrow.”
Schaper took advantage of the par five second for the third day in a row before making pars at the next 12 holes.
He then followed up a beautiful approach into the long 15th by rolling in his eagle putt from eight feet to jump to 11 under.
After holing his 25 foot birdie putt on the 17th, the talented youngster bravely took on the flag at the 18th, clearing the water before knocking in his birdie putt to get to 13 under.
Schaper was delighted with the way he finished the round – particularly his eagle on the 15th. He said: “My tee shot came down the right side of the fairway which, for that flag, kind of just sets it up.
“We picked the club and we just trusted it. I was really chuffed with that golf shot. I’m feeling really good with the swing at the moment.
“To hit a shot like that in a tournament like this is really cool.
“Throughout the tournament I’ve been playing the last four holes pretty solid. From the 15th, 16th, 17th and 18th, I really enjoy them.
“They kind of suit my shape. I love coming down the stretch knowing that I can possibly birdie them. The 16th’s a pretty tough hole, the 17th also can play a really tough hole.
“So I know that if I get through these holes and make a nice score here I can definitely get myself closer to the top.”
European Tour Release, Nov 28, 2020