In the first match, 267th ranked Ramanathan overcame Temur Isamilov 6-2 5-7 6-2 7-5, while debutant Gunneswaran, ranked 187, outwitted Sanjar Fayzieb 7-5, 3-6, 6-3, 6-4.
Ismailov is ranked 406, while Fayzieb was played in place of Denis Istomin, who had pulled out of the second round tie due to a foot injury.
To his credit, Ismailov made life difficult for the Indian despite suffering cramps in his right hamstring.
The Indian sealed the opening set in a jiffy and was leading 4-3 in the second when he saw himself down 0-40. He saved two breakpoints but netted a backhand on the third to allow Ismailov stay alive.
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The Uzbek played freely while Ramkumar put himself under pressure to finish the job quickly. The nerves got to Ramkumar and, in utter desperation, he squandered four set points and was eventually broken in the 12th game.
Ismailov, though, recovered remarkably after a medical timeout and was battle ready for the fourth set. Ramkumar was always catching up with the Uzbek, holding his serves without fuss, and soon it was 5-5.
Two unforced errors by Ismailov gave Ramkumar three break chances. He converted the third when Ismailov buried a return to the net.
Ramkumar's unforced errors continued as twice he sent down double faults on match points. He though sealed the match with a forehand volley winner.
In the second set, Gunneswaran trailed Fayziev 1-4 and the Uzbek levelled the match 1-1.
Fayziev led the third set 3-1 as Gunneswaran committed some silly errors, which made his task even more difficult.
But, to his credit, Gunneswaran refused to get bogged down and raced past the Uzbek to take a 4-3 lead.
Gunneswaran won the third set, and led the tie 2-1, before he overwhelmed Sanjar 6-4 in the fourth set and sealed the match in his favour.
Ramkumar later said encouraging words from Leander Paes and Rohan Bopanna helped him in getting India off to a winning start.
Bopanna too, "likewise", offered him an advice.
The winner of the tie will qualify for the World Group play-offs, to be played in September.