South Africa has three left-handed batsmen in its top-6 batting line-up: Quinton de Kock, Ryan Rickelton and David Miller. Being an off-spinner, Sundar takes the ball away from these batsmen, thereby creating wicket-taking opportunities. De Kock and Rickelton are known for explosive starts, while Miller plays the role of finisher. Getting any of these out early will put pressure on South Africa.
According to cricmetric.com, Sundar has taken 26 wickets in 50 T20I innings against left-handed batsmen. During this period his economy rate has been 6.84. Against left-handed batsmen, 49.7% of his balls are dot balls, which shows his ability to stop runs.