Dimitrov won the match 6-7 (2), 6-4, 4-6, 6-4, 10-8 on court 18 at Roland-Garros in Paris.
Donaldson started getting cramps in the fifth set of his matchup against his fourth-seeded opponent. The American held the ball in his left hand and dropped it down to his racket, before swatting it over the net underhand. He won a point after the service, as Dimitrov over hit the return.
He tried the serve again later in the match, but it didn't go quite as well. Donaldson's serve went over the net, but he hit the net on his return.
The match lasted four hours and 20 minutes. After match point, Dimitrov shouted and kissed his knees.
"I'm feeling good right now to be honest," Dimitrov told the Tennis Channel after the win. "After playing four 4:20. It was a great battle. He definitely knew what he wanted to do today. I was not surprised."
Dimitrov battles Spain's Fernando Verdasco at 5 a.m. Wednesday in the round of 32.