Ishan Kishan and Shreyas Iyer put on 161 runs for the third wicket.
Chasing 279 to win, the hosts rode on Shreyas Iyer’s unbeaten century (113 off 111 balls) and Ishan Kishan’s 84-ball 93 to overhaul the target with 4.1 overs to spare.
"The toss worked perfectly, I am glad. Thanks to Keshav that he chose to bat (laughs). I must say that Ishan and Shreyas, the way they created that partnership was great to watch," said Dhawan in the post-match presentation ceremony.
South Africa’s stand-in skipper Keshav Maharaj’s decision to bat first after winning the toss turned out to be an advantage for the Indian team as the dew factor made things easier for the batters in the second half.
"The ball was coming on nicely but it was keeping low. So our plan was to take on the bowlers in the first ten overs. But once the dew came into the picture, it was skidding on. So the back-foot shots were easy to execute," said Dhawan.
Earlier, the Proteas posted 278/7 in their 50 overs on the back of Aiden Markram’s 89-ball 79 and Reeza Hendricks’ 76-ball 74.
Pacer Mohammed Siraj was the stand-out bowler for India, claiming three wickets by conceding 38 runs in his 10 overs. Slow left-arm spinner Shahbaz Ahmed, who made his ODI debut in Ranchi, also returned with impressive figures of 1/54 in his 10 overs.
"I am very pleased with the bowlers, especially Shahbaz, the way he bowled in the first ten overs and got us the breakthrough," said Dhawan.
The series-deciding third and final ODI will be played in Delhi on October 11.
(With ANI inputs)