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Venter was never in this fight as the cagey Mchunu first outwitted and then out-punched the taller veteran, who earlier in this competition had beaten Daniel Bruwer and top seed Zack Mwekassa.
Mchunu had won his two bouts by scoring first-round stoppages over Flo Simba; it took him a little longer to beat Venter.
Both were cautious in the early rounds, but Venter made the mistake of pressing the attack, and getting countered by his shorter opponent.
Against Bruwer and Mwekassa, Venter had played counter-puncher. Against Mchunu - against whom he had dropped an eight-round points decision last year - he decided attack was the best option.
Bad move - that strategy turned him into a walking punch bag.
Southpaw Mchunu had the superior hand speed, and he landed his lightning left every time Venter tried to close the gap.
And stubborn Venter refused to veer from his strategy of pressing the attack and trying to land punches on a small and awkward target.
It was like a bazooka trying to hit a mosquito, and as the first three rounds ticked by, the little guy notched up the points - in the fourth round he unleashed the heavy artillery.
Mchunu landed a succession of hard lefts and a right that left Venter reeling on the ropes. Early in the fifth round Venter still tried pushing forward and he was dropped by a left.
He beat the count but went down voluntarily twice more, and the referee put him out of his misery by waving the fight over.
Venter was a beaten man.
I had thought Venter would win; how wrong I was. On the night he was no match for Mchunu, who won 60% of a R500,000 pot as well as the African Boxing Union cruiserweight title.
For Venter, consolation came from an early night and a R200,000 purse.