As it happened: Zimbabwe v SA
De Kock wasted little time in his 75-ball innings and was aided by 40 off 33 balls by stand-in captain Faf du Plessis as the Proteas chased down a target of 166 in just 27.2 overs.
The heavy defeat came after both sides made three changes, with the most notable arguably being Zimbabwe's decision to drop their premier batsman Brendan Taylor.
Taylor had scored just 20 runs in the first two games, but his omission still came as a surprise to the South Africans.
"Obviously you do a lot of planning as a captain to someone like that. He's a quality player so I was very happy not to see him," Du Plessis said.
Although Elton Chigumbura filled the number four berth admirably with a captain's knock of 90, he found next to no support as Zimbabwe were bowled out inside 40 overs.
After Kyle Abbott had trapped Hamilton Masakadza lbw in the first over, Marchant de Lange struck with his very first delivery when he had Richmond Mutumbami caught at second slip.
Chigumbura revived the innings alongside Sikandar Raza Butt, who made 30 from 24 balls, but after Raza was dismissed by Ryan McLaren, the Zimbabweans went on to lose six for 16 in six overs.
With Zimbabwe 119 for nine, Chigumbura went to his half-century in 102 deliveries, and then cashed in during the power play, hitting five fours and two sixes as he added 40 in 20 balls.
That took him past his previous best of 79 -- scored against Kenya in Mombasa in 2009 -- but his fun came to an end when he was bowled by De Lange, who finished with 3 for 31.
South Africa suffered an early blow when debutant Rilee Rossouw was brilliantly run out by Malcolm Waller as he attempted to get off the mark, but De Kock and Du Plessis added 73 in fewer than 12 overs.
"I think when you're chasing you want to put the pressure back onto the opposition," said Du Plessis.
"I assessed the wicket and I felt it was a much better one to bat on, so I felt I could play a little more freely and it was nice to bat like that."
Although the partnership was ended when Du Plessis was bowled by Sean Williams, De Kock went to fifty and then opened up, taking 32 runs from his next 12 deliveries.
The left-hander's big hitting eventually caught up with him when he found deep midwicket, but JP Duminy and David Miller soon finished the job for South Africa.
De Kock was named man of the match, and also scooped the man of the series award for his 185 runs in the three matches.
two sides now return to Harare for a triangular series that also
involves Australia, who flew into the country on Thursday afternoon.
Zimbabwe: 165 in 39.5 overs (Elton Chigumbura 90; Marchant de Lange 3-31)
South Africa: 171/3 in 27.2 overs (Quinton de Kock 84, Faf du Plessis 40; Sean Williams 1-1)
Result: South Africa win by seven wickets