After conceding its highest total in ODIs, England was dismissed with 28 overs to spare in its heaviest defeat by runs in ODIs.
Klaasen hit his first World Cup hundred, and fourth overall, and it was the second quickest hundred in this tournament, after compatriot Aiden Markram’s 100 off a record 49 balls against Sri Lanka. Klaasen hit 12 fours and four sixes overall.
The big win cements South Africa’s third spot with six points. England sank to ninth in the table, tied on two points with last-place Afghanistan.
“We have been playing in India for quite a few years now, so we are used to the conditions. We had a good practice session here before the game and knew how to use the shorter boundaries,” said Klaasen, who was player of the match.
“Losing to the Netherlands was a big low for us. You have to dig deep after a loss like that, and playing for my country is an honor. I have worked my whole life towards it and it is a great moment.”
Put into bat, South Africa lost Quinton de Kock for four runs. Reeza Hendricks, coming in for skipper Temba Bavuma who was out because of illness, scored 85 off 75 balls. He hit nine fours and three sixes.
Hendricks reached 50 off 48 balls, and put on 121 runs off 116 deliveries with Rassie van der Dussen, who scored 60 off 61.
Adil Rashid pulled things back for England. Van der Dussen was caught at midwicket, and then Hendricks chopped a googly onto his stumps. It brought stand-in skipper Markram and Klaasen to the crease.
They added 69 runs for the fourth wicket, before South Africa lost its way again. Markram was caught behind in the 35th off Reece Topley, who came on to bowl after receiving treatment on his injured left hand. The left-arm pacer then had David Miller caught for five two overs later.
At 243-5, it seemed South Africa would have to settle for a sub-par total on a good batting surface. Klaasen though had other plans.
Struggling with intense sweating under high humidity and suffering from cramps, Klaasen wasn’t able to run much. He made up for it with fours and sixes instead, as South Africa crossed 300 in the 44th over.
Only four overs later, South Africa passed 350. Klaasen, who scored 50 off 40 balls, raced to three figures off only the next 21 balls. At the other end, Jansen scored 50 off 35.
Klaasen and Jansen helped South Africa plunder 143 runs in the last 10 overs in an incredible display of power hitting, smashing 15 fours and 10 sixes during their stand.
Chasing a mammoth 400, the English innings never took off. Three of its top four batters didn’t even reach double figures.
Jonny Bairstow holed out for 10 off Lungi Ngidi (2-26), while Dawid Malan was caught behind off Jansen.
Jansen enjoyed a productive day with the ball too – he picked up 2-35 in five overs, also sending back Joe Root for two runs. Root hit the ball straight off his pads to the leg gully for a simple practice catch.
The game seemed over when the inspirational, fit-again Ben Stokes (5) offered a simple return catch to Kagiso Rabada, who gleefully accepted it.
The procession of wickets continued. Gerald Coetzee sent back skipper Jos Buttler (15) and Harry Brook (17) in the 12th over. Buttler was caught behind, while Brook was out lbw, as England was down and out at 68-6.
Coetzee finished with 3-35. Mark Wood’s 43 not out off 17 balls was the innings’ top score.
Gus Atkinson hit 35 in a bid to reduce the startling deficit.
“This loss leaves us in a tough position,” Buttler said. “There is no room from here on. It is going to be incredibly difficult (to qualify for the semifinals). We haven’t left ourselves any margin, but we have belief.”
Both teams are coming off shock defeats - England to Afghanistan and South Africa to Netherlands.