Chasing 165, Washington slipped to 29-3 before Welkom-born Andries Gous and Nikhil Chaudhary rebuilt the innings with a 65-run stand.
Gous made 40 off 35 balls, but his dismissal left Washington struggling at 109-6 after 15.4 overs.
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Pienaar then launched a remarkable counter-attack, smashing 53 off just 24 balls, including three fours and five sixes. He brought up his half-century from 23 deliveries and dragged Washington to the brink of victory.
Van Schalkwyk struck with the second ball of the final over to remove Pienaar before dismissing Ian Holland with his next delivery. Washington still had a chance to force a Super Over, but Lahiru Milantha was run out from the final ball as they finished on 163-9.
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Van Schalkwyk conceded 50 runs across his four overs but claimed two crucial wickets and played a part in the decisive run-out.
Earlier, South African-born Matthew Tromp hammered 39 off 21 balls to help Los Angeles post 164. His innings included three fours and three sixes as the Knight Riders secured the MLC title in dramatic fashion.
Photo: Cognizant Major League Cricket/X
