Greece's Maria Sakkari beat defending champion Iga Swiatek to reach her maiden Grand Slam semi-final at the French Open at Roland Garros.
Sakkari, the 17th seed, is now the highest-ranked woman in the draw after seeing off Swiatek 6-4 6-4.
"I am speechless, it is a dream come true," said the 25-year-old. "It is a very nice feeling."
Krejcikova, meanwhile, saved five set points from 17-year-old Gauff in the opener before the world number 33 went on to seal victory on her sixth match point.
"I never imagined I would be standing here one day," said Krejcikova.