Bulgarian Dimitrov, a semi-finalist at Melbourne Park last year, fell in a huge hole in the fourth set against the free-swinging American, whose shot-making and composure made a mockery of his humble ranking.
Dimitrov had to wait and hope for McDonald to falter under the lights at Rod Laver Arena, and finally the 22-year-old American did with a double-fault that produced match point.
McDonald went for broke with a forehand to save it, but slammed into the net, handing the match to Dimitrov who roared in relief.
Dimitrov's previous five-set match at Rod Laver Arena ended in heart-break, a defeat to Rafa Nadal denying him a maiden grand slam final.
So it was understandable during post-match media duties that he was keeping positive about his great escape against McDonald.
"It's just a hard day, I'm admitting it, I couldn't even hide it," the 26-year-old said after setting up a match with Russian 30th seed Andrey Rublev.
"I landed on the good side of things today. In past years maybe I would have lost that match."
McDonald, who had never played anyone higher than world number 69, exited to a warm ovation, having rocked his opponent with aggression and out-thought him with strategy.
Rublev may be buoyed by the young American's gallantry but also by the wavering of Dimitrov, who has laboured for years with a reputation greater than his achievement.
Expectations he could challenge for the title this year seemed to weigh heavy on his broad shoulders, particularly in the fourth set when his game fell apart.
He had never lost to a qualifier at a grand slam, let alone a player ranked lower than 95, but found himself dragged into a marathon scrap against a rookie of only four tour-level events.
NERVOUS PLAY
Dimitrov played the fourth set as if victory was his birthright, swinging lazily at balls that sailed long and wide.
In a trice, he was down a double-break and throwing nervous glances at his coach Dani Vallverdu in the players box.
Though clad in loud pink from head to toe, Dimitrov retreated into his shell, and hammered a 13th unforced error for the set while McDonald feasted on a string of weak second serves to march into the fifth set with all the momentum.
Dimitrov grafted hard to hold serve, halting a six-game losing streak, but his game remained little more than hit-and-hope.
He would have given his serving arm for McDonald to fade, but the American dragged the match deep with superb composure, keeping up the firepower to the last.
However, serving to stay in the match at 7-6, McDonald finally blinked, his only double-fault of the set committed at the worst possible moment.
He was never going to die wondering, and he moved in to wind up for a raking forehand that pounded into the net rather than save the match point.
"There are a lot of emotions going on," McDonald said.
"On one hand I'm pretty excited how I competed tonight... on the other hand I know how close I was to winning, too. That kind of sucks, but he's good player, he's been out for a while.
"Overall, I'd say there's so many more positives than negatives."
(Editing by Christian Radnedge)
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