The 373-ranked Chennai lad made his Russian rival, ranked 100, work hard for points in his first ever Challenger level last-eight match, which he lost 6-7(8) 6-7 (5) after sweating it out for two hours at the Centre Court of the Balewadi Sports Complex.
Prashanth, who won three ITF Futures this season, proved that his victory over Somdev Devvarman was not a fluke.
He did battle initial nerves but then took off and presented a strong fight against second seed Donskoy, who came into the tournament after beating top-100 players, including world number 39 Borna Corcis, at the Kremlin Cup which is a ATP 250 event.
He backed his serve with decent ground-strokes and little more power in his shots would have added extra sting to his challenge.
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The 29-year-old, who trains with Mahendra Verman in Chennai, said three points made all the difference.
"In the first set tie break, I was serving at 5-5, I could not go up and at 7-7 too. In the second set also at 5-5 I could not go up and that made a difference. I wont say that I would have won the match but had I gone up, it would have given me a better chance," Prashanth said after the match.