The 28-year-old's rich vein of form continued as he looked in complete command for his 108 off 195 balls, having made a sublime 215 in the series opener in Visakhapatnam last week.
India vs South Africa, 2nd Test, Day 3: Ashwin's four-for folds Proteas on 275; India lead by 326 runs
India vs South Africa, 2nd Test Day 2: Virat Kohli's career-best puts Proteas on the mat
IND vs SA, 1st Test: 'Five-star' Shami, brilliant Jadeja star in big India victory
IND vs SA, 1st Test, Day 3: Ashwin's fifer puts hosts on top after Elgar, De Kock centuries