Czech Karolina Muchova overcame illness to reach her second consecutive US Open semi-final with a 6-1 6-4 win over Beatriz Haddad Maia.
Muchova left the court after wrapping up the first set with ease, ran off court at 2-1 and again at 3-2 in the second and was seen by the doctor and physio at 4-3.
The unseeded left-hander, who only returned to action in June after having wrist surgery in February, will face either top seed Iga Swiatek or American sixth seed Jessica Pegula for a place in Saturday's final.
"It was a weird match," said Muchova.