Slovakia's 48th-ranked Daniela Hantuchova advanced to the US Open quarter-finals with a rain-interrupted 3 sets victory over 81st-ranked American wildcard Alison Riske.
Hantuchova, a 2008 Australian Open semi-finalist in her best Grand Slam run, has matched her best US Open showing from 2002 by reaching the final eight.
The 30-year-old will next face either second seed Victoria Azarenka of Belarus or Serbian 13th seed Ana Ivanovic, whose match was postponed until Tuesday after showers halted play for more than four hours.
Unseeded Italian Flavia Pennetta reached the US Open quarter-finals for a fourth time with a straight sets win over Romanian 21st seed Simona Halep.
Pennetta will next tackle 10th-seeded compatriot Roberta Vinci with a place in a Grand Slam semi-final for the first time at stake.
Earlier, Italian 10th seed Vinci beat compatriot and qualifier Camila Giorgi to reach the US Open quarter-finals for a second successive year.