World number 82 Pella had high hopes of progressing after his five-set upset victory over third seed Marin Cilic of Croatia on Thursday.
But the 23-year-old McDonald, ranked 103, was on a roll after coming through the qualifiers before reaching the third round with a five-set victory over Chile's Nicolas Jarry that went to 11-9 in the deciding set.
McDonald will not know his opponent in his first Grand Slam fourth round match until Saturday after the clash between 13th seed Milos Raonic and Austrian Dennis Novak was suspended at one set all with the Canadian leading 6-5 in the third.
A disappointed Pella said: "If I'd played Cilic the way I played today I'd have lost in an hour. It's all about how I feel facing each match and today I was tense and couldn't loosen up."
(Reporting by Rex Gowar; Editing by Ken Ferris)
