Markram has played a key role in South Africa's unbeaten run to the semi-finals, scoring 268 runs in seven matches at an average of 53.60 and a strike rate of 175.16 - the best among openers who have faced at least 100 balls in the tournament.
Du Plessis pointed to Markram's 2025 season with Lucknow Super Giants as the turning point, when he was asked to open the batting.
"The IPL almost demanded that you play that way to keep your place," Du Plessis said on ESPNcricinfo's TimeOut show. "With the Impact-Sub rule and the way the game has moved, an opener can't play the old-school way anymore. If you didn't attack in the first six overs, someone else would."
During that campaign Markram posted a powerplay strike rate of 151.13 and finished the season at 148.83, outscoring several established openers including Quinton de Kock and Rohit Sharma.
Previously a middle-order batter for both South Africa and Sunrisers Hyderabad, Markram has since been promoted permanently to the top of the order, with Ryan Rickelton moving to No.3 in the current tournament.
Du Plessis said the change highlights the importance of versatility in modern T20 cricket.
"Aiden realised he had to increase his strike rate," he said. "That move from No.3 or 4 to opening has worked really well for him."
South Africa will hope their in-form captain continues that form when they face New Zealand national cricket team in the first semi-final at Eden Gardens on March 4.
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Proteas captain Aiden Markram cut a dejected figure as he fronted up to the press in the wake of their T20 World Cup semifinal loss to New Zealand on Wednesday.
