Australian captain Steve Smith was dismissed for a duck in controversial scenes to bring South Africa back into the contest in the first Test.
Smith was given LBW by field umpire Aleem Dar from the bowling of spinner Keshav Maharaj, but it was immediately reviewed by the Aussie skipper.
He could not believe he was given out after dancing several strides down the pitch to meet the ball.
The Decision Review System supported Dar's call. The predictive system judged only part of the ball would have his Smith's leg stump, but it was enough for the DRS to deliver a verdict of umpire's call.
Keeper Quinton de Kock hardly appealed initially and Smith immediately reviewed the verdict, but ball-tracking replays deemed it was an "umpire's call".
It's likely the classy right-hander would have remained at the crease under old Decision Review System guidelines on how much of the ball needs to hit the stumps, but those rules were changed this year.
"That's remarkable. That's an incredibly brave decision from Aleem Dar ... I can understand where he's coming from but he's so far down the wicket," Kevin Pietersen said in the Nine Network commentary box.
Michael Clarke agreed.
"You would assume that ball is going to bounce over the stumps," he said.
"You can definitely understand Steve's disappointment. He's down the wicket at a ground like the WACA."
Smith's dismissal left Australia 3/168.
Usyk won the fight by unanimous decision, with all three judges scoring it 116-112.