While Somdev overwhelmed Izak Van der Merwe 7-6 (5), 6-3, 6-4 in the first singles, Bopanna rallied from a set down to beat South Africa's No 1 Rik De Voest, ranked 189, 2-6, 6-4, 6-2, 6-4 and put India 2-0 ahead.
Victory in Saturday's doubles rubber will clear India's entry into the World Group after 11 years.
Bopanna refused to throw in towel despite being drubbed in the first set and turned the tables on his higher ranked opponent.
He struggled to get on the board initially, but raised his game by several notches as the match progressed.
Like Somdev, he too took a while to get into rhythm and never looked back after breaking Rik in the fifth game of the second set.
After making it one set all, Bopanna played an amazing game as he served extremely well and his booming single handed backhands and forehands were too hot to handle.
Coming into the tie after his breakthrough performance at the US Open, the 133rd-ranked Somdev played with maturity and took two hours and 10 minutes to dispatch his opponent at the Ellis park indoor arena.
Rohan Bopanna, India's number two in the tie, will now clash with South Africa's number one Rik De Voest, ranked 189 in the world, in the second singles.
Somdev faced just one breakpoint and mostly played from the baseline. Merwe displayed an excellent serve and volley game but unforced errors dented his chances.
Somdev took little time to settle as he struggled with his first serve and sprayed a lot of forehands long.
Merwe, ranked 289 in the ATP chart, on the other hand was confident in the start, played some big strokes and won brilliant points at net.
No player got the break opportunity until eighth game of the opening set. Merwe set up the breakpoint with an overhead volley but Somdev came back well with strong serve to save.
India got a break point in the next game but Somdev was unlucky as he fired a backhand, eyeing a cross-court winner, but the ball landed on the net and jumped out of the court. Without seeing a break, the first set stretched to the tie- breaker, which the Indian won to go 1-0 up.
Merwe, who looked better player initially because of his exploits at the net, backed by a solid serve, succumbed to unforced errors in the tiebreaker.
The South African hit a forehand to net to give Somdev a 3-1 lead, which he later reduced to 4-6 and saved a set point too but his double fault at 5-6 sealed the set for India.
Somdev curbed his unforced errors in the second set and broke Merwe in the sixth game to be in advantageous position. A furious backhand, after four deuce points, earned Somdev his second breakpoint and with equally forceful double-handed backhand he got that break for a 4-2 lead.
Merwe started to wilt under pressure and handed two set points in the eighth game, thanks to some unforced errors. Pressure got the better of Merwe and he was broken at love in the fifth game at love when he double faulted.
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