Daria Kasatkina booked her place in her first Grand Slam semi-final with a 6-4 7-6 (7-5) win over her fellow Russian Veronika Kudermetova.
Kasatkina, the 20th seed, took the first set against the run of early aggressive play from Kudermetova.
Both players traded breaks of serve in an edgy second set, with Kasatkina winning on her fifth match point after overcoming nerves in the tie-break.
Kasatkina will face either Iga Swiatek or Jessica Pegula in the last four.
"You could see the match was really nervous and tight, especially the tie-break, but I am happy at the end I won," Kasatkina said in her on-court interview.