Ramkumar was leading the contest 3-6 6-4 4-1 when Chinese qualifier Yan Bai retired due to a left-knee issue.
The 20-year-old Ramkumar now faces Myneni, who is ranked ranked 23 places above him at 262.
Myneni had shocked top seed and world number 85 Ricardas Berankis in his first round.
Wild card entrant Sanam Singh bowed out from singles with a 3-6 4-6 defeat against Indonesia's Christopher Rungkat.
Sanam though advanced to the doubles quarterfinals with partner Myneni. They beat Fabrice Martin of France and India's Purav Raja 4-6 6-3 10-4 in their first round.