Du Plessis, who missed the team's last league game due to over-rate offence, replaced Farhaan Behardein in the lone change.
India retained the same Playing XI from their previous game, which meant Shikhar Dhawan and Mohammed Shami missed out on this big game.
Captain MS Dhoni, at the toss, said he too would have like to bat first had he won the toss. India, the 2007 champions, are unbeaten so far in the tournament.
The winner of this game faces Sri Lanka in the final on Sunday at the same venue.
India: Mahendra Singh Dhoni (captain and wicketkeeper), Rohit Sharma, Virat Kohli, Yuvraj Singh, Suresh Raina, Ravindra Jadeja, Ravichandran Ashwin, Bhuvneshwar Kumar, Mohit Sharma, Ajinkya Rahane, Amit Mishra.
South Africa: Faf du Plessis (captain), Hashim Amla, Quinton de Kock (wicketkeeper), AB de Villiers, JP Duminy, David Miller, Albie Morkel, Dale Steyn, Imran Tahir, Beuran Hendricks, Wayne Parnell.