Wehrlein had started in sixth ahead of Buemi, but lost a position on the opening lap as the Swiss dived down the inside into the first corner and held the position.
As he tracked Buemi along the high-speed stretch along the Mouille Point Beach, Wehrlein locked up on the entry into Turn 10 and went into the back of the Envision driver, who spun and went backwards into the Tecpro barrier.
For his part, Wehrlein's Porsche proved too damaged to continue and the German ended the race unable to extend his championship lead.
"It was clearly my fault," Wehrlein told Autosport. "I braked a bit too late, and when I realised that I was braking too late, I tried to brake even harder and then just locked up.
"Then I couldn't avoid the accident anymore, unfortunately. It was definitely my mistake and I can only apologise [to Buemi].
"It's a lap 1 thing: cold tyres, lots of people around. Like I said, I just misjudged the breaking point and therefore couldn't avoid him anymore."
Buemi was able to continue, albeit in last, but worked his way back up the order and made up three positions in the final two laps to claim a fifth-placed finish.
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Although Wehrlein failed to finish, the German's championship lead remained undiluted as main rival Jake Dennis also failed to score as a result of a drive-through penalty, following a tyre pressure violation.
Without his first-lap crash, Wehrlein reckoned he could have been on to win given that team-mate Antonio Felix da Costa claimed victory in South Africa from 11th on the grid.
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"I would say so, yes," he stated in response to a question asking if he could have replicated da Costa's result himself.
"I mean, I was starting P6, so yeah, we were up for a good race. But there are no ifs and what-ifs in motorsport.
"We go home with zero points, but the good thing is that in a few weeks' time, we have already the chance to come back stronger."
