Klaasen hammered 174 off just 83 balls at his home ground in Centurion to help the hosts to a total of 416/5, and eventually a 164-run victory that sees the five-match series level at 2-2 with one match left to play.
Walking to the crease in the 26th over with the score on 120-3, Klaasen went on to strike 13 fours and as many sixes in a brutal display of power-hitting that stunned Australia's bowling attack.
The 32-year-old said he gave himself time at the start of his innings, he only scored two boundaries in his first 26 balls, to asses the conditions before going on his rampage.
"I am batting within myself at the start to make sure I get a good platform and then I can just react to every ball: stand still, watch the ball and wherever I need to hit it, my body will take over and just react," Klaasen revealed after the game.
"It was awesome out there and you don't often get that feeling - maybe once or twice in your career."
Klaasen has gone on to become one of the best batters in white-ball cricket with him being courted by several T20 franchise teams around the world.
He described the mindset adjustment that transformed his game back in 2021 when he was dropped from the Proteas side and returned to his domestic team.
"The coaches said to me, 'You are using too many options. Let's limit yourself'. It took me a couple of months to get back to my best and a lot of hard work," Klaasen added.
"What's changed in my career is that I am playing every ball as it is and for me to stay in that mindset, I don't recap what I have done the previous ball or think of what I might do the next ball."
Klaasen will hope to continue his form in the fifth and deciding ODI that takes place at the Wanderers on Sunday.