Ferrer had the match nearly wrapped up at 6-3, 3-1 on Tuesday, with Donskoy serving at 15-40, then everything changed, reports Efe.
The 26-year-old Donskoy managed to hold serve and break Ferrer to even up the match at 3-3.
The Russian world No. 84, who has not won any titles on the ATP World Tour, had lost to the 34-year-old Ferrer, who is ranked 12th in the world, at the French Open earlier this year.
Ferrer, who appeared rattled, was broken late in the third set and Donskoy held on to win.
Donskoy will play the winner of the match between Portugal's Gastao Elias and American Steve Johnson.
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