Debutant Dawid Malan hit 78 from 44 balls to help fire England to 181/8 from their 20 overs. It was the highest-ever IT20 score by an England debutant.
The hosts then restricted the Proteas to 162/7 to win by 19 runs and clinch the series 2-1.
Chris Jordan was the pick of the bowlers with figures of 3/31 while Tom Curran impressed again, taking 2/22, and young leg spinner Mason Crane dismissed AB de Villiers for his first-ever international wicket.
The Middlesex big-hitter started as he meant to go on, powering 12 fours and two sixes in his outstanding 44-ball 78. Rocking onto the back foot to slap Imran Tahir straight down the ground and past the fence, it was easy to wonder how he’d not yet featured for England at 29 years old.
Malan continued to hit out powerfully, carving Andile Phehlukwayo behind square for four more before driving up and over Mornie Morkel’s head, way over the sight screen and into the River Taff.
He brought his half-century up with a clip to the fine leg boundary off Tahir and looked well set to go onto an even bigger score before miscuing a big swipe down the ground off the same bowler, Dane Paterson holding the catch to dismiss him for an unforgettable 78.
Hales had taken his time to get settled but worked through a range of big hits, including a hammer blow straight down the ground for six, before struggling to get hold of a pull and falling to a very good low, diving David Miller catch for 36.
After Man of the Match Malan departed in the fourteenth over, England struggled to form another significant partnership. Phehlukwayo and Paterson both bowled supremely at the death, going through all their variations to surprise the batsmen as they periodically took wickets throughout the final overs.
Stand-in skipper Jos Buttler enjoyed a quickfire cameo of 31 off 22 balls but Sam Billings, Liam Livingstone, Liam Plunkett and David Willey all departed for 12 or under as the death bowlers turned the screw.
The target England had to defend to win the game and the series was set at 182.
England’s bowlers enjoyed a dream start, Curran trapping Reeza Hendricks with his fifth ball as the opener lofted up a simple catch to Plunkett at mid-off.
The good work continued when Jordan had surprise number three batsman Chris Morris caught brilliantly by a leaping Hales on the deep square leg boundary for eight. JJ Smuts was the next to go, top-edging Plunkett’s short delivery down the throat of Malan for 29.
Still, dangerman de Villiers remained at the crease and he’d clearly decided to go after leg spinner Crane, playing in only in his second England match. After hitting the 20-year-old for three consecutive boundaries, including two huge swept sixes, Crane held his nerve and the Proteas skipper slapped the ball to Hales at mid-wicket. Cue wild celebrations!
Miller and Fabien Behardien were next to fall, both to the pace and bounce of Jordan who bowled with great skill and control for his three wickets. Before Mangaliso Mosehle and Phehlekwayo shared a battling 54-run stand to give South Africa an unlikely chance.
It wasn’t enough for the visitors as England ran out deserved 19-run victors to clinch the series 2-1.