Iga Swiatek said she was "really proud" to reach her first US Open quarterfinal but the world No 1 was forced to dig deep to beat Jule Niemeier in the fourth round on Monday.
French Open champion Swiatek recovered from a set and a break down to see off her 108th-ranked German opponent, 2-6, 6-4, 6-0.
Top seeded Swiatek will face Jessica Pegula for a place in the semi-finals after the American eighth seed cruised past two-time Wimbledon champion Petra Kvitova 6-3, 6-2.
In a mistake-riddled clash on Louis Armstrong Court, there were a total of 15 double faults, 12 breaks of serve and 76 unforced errors. "It's really satisfying.
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