Ranked 267, Ramkumar eked out a 6-2 5-7 6-2 7-5 win over the Uzbek, who is ranked 406, in the opening singles rubber of the second round tie.
Prajnesh Gunneswaran, ranked 287, makes his India debut against Sanjar Fayziev (376).
To his credit, Ismailov made life difficult for the Indian despite suffering cramps in his right hamstring.
Ramkumar dominated the proceedings before a bout of double faults handed Ismailov a small opening, which the Uzbek garbbed with both hands to make a match of it.
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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.
The Uzbek had snatched the momentum but suffered cramps in his right leg in the third set, making the job of the Indian easy. Ramkumar ran away with the third set as Ismailov's restricted movements gave him easy points.
Two unforced errors by Ismail gave Ramkumar three break chances. He converted the third when Ismail 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.
The winner of the tie will qualify for the World Group play-offs, to be played in September.