Krunic dropped the first nine games of the match and was down 4-1 in the second set before prevailing 0-6, 6-4, 7-6 (4).
In another stunner, fifth-seeded Petra Kvitova was upset by unseeded Daria Gavrilova of Australia 6-2, 6-1.
Gavrilova saved six of seven break points and broke serve six times to oust the two-time Wimbledon champion.
Last year's semifinalist, Jelena Ostapenko of Latvia, beat Magdalena Rybarikova 6-4, 7-6 (3), while No. 10 seed Julia Goerges of Germany beat Johanna Konta 6-2, 4-6, 6-3 to advance.
Donna Vekic of Croatia and Danielle Collins of the United States also moved on.
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