Both sides scored three tries, but Du Plessis (inside centre) was the match-winner for Griquas with his laser-like aim from the tee.
Du Plessis punished the ill-disciplined Pumas at regular intervals with his prolific boot. The visitors received four yellow cards, with Daniel Maartens (loose forward) getting a red-card for collecting two yellows.
In the first half, Tafel Lager Griquas had the Airlink Pumas in all sorts of trouble with their powerful scrum and rolling maul, resulting in the Mpumalanga outfit conceding a bag full of penalties, having two men sin-binned and conceding a penalty try.
The men from Kimberley, who played well in their previous two matches but finished on the losing side on each occasion, were desperate for the win on their home ground. They dominated the forward exchanges but could not really pull away from the visitors.
Alandre van Rooyen (hooker) scored near the end of the first half while Du Plessis kicked two penalty goals. Corne Fourie (hooker) and Jade Stighling (wing) scored for the visitors while pivot Tinus de Beer added a penalty goal as Griquas went into the halftime break with a 21-13 advantage.
With the stiff breeze in his back and his forwards dominating proceedings, Du Plessis added six more penalty goals in the second stanza, one of them a spectacular effort from more than 60m out, which capped a great performance by the inside centre.
The Pumas' second-half try was scored by replacement prop Simon Raw, which Eddie Fouche (replacement flyhalf) converted, while Munier Hartzenberg (wing) added a third five-pointer for the hosts.
Tafel Lager Griquas 41 (21) - Tries: Alandre van Rooyen, Munier Hartzenberg; Penalty try. Penalty goals: Zander du Plessis (8).
Airlink Pumas 20 (13) - Tries: Corne Fourie, Jade Stighling, Simon Raw. Conversion: Eddie Fouche. Penalty goal: Tinus de Beer