Teenager Belinda Bencic's fairytale US Open run continued on Sunday as Caroline Wozniacki also advanced.
Bencic, 17, was too good for Serbian ninth seed Jelena Jankovic in a 7-6 (8-6) 6-3 fourth-round win at Flushing Meadows.
It continued the Swiss teen's best ever grand slam result, and the draw could see her go even further.
Bencic, who hit 30 winners in her victory, faces Peng Shuai next after the Chinese woman upset Czech 14th seed Lucie Safarova 6-3 6-4.
A two-time junior grand slam champion, Bencic saved three set points in the first set.
She coughed up a 4-1 lead in the tie-break as Jankovic rallied, but Bencic converted her third set point chance.
After breaking in the fifth game of the second set, Bencic was always in control and showed no nerves as she advanced.
Peng, meanwhile, was consistent during her win over Safarova, hitting 17 winners and just seven unforced errors.
Safarova had won the previous four meetings between the pair, but that mattered little on Sunday.
On the same side of the draw, Wozniacki and Sara Errani advanced.
Danish 10th seed Wozniacki overcame Russian fifth seed Maria Sharapova 6-4 2-6 6-2 in a good performance.
Wozniacki, among the outsiders for the title after a good year, capitalised on what was a rusty tournament from Sharapova.
She will face Errani in the quarter-finals after the Italian 13th seed beat Croatian veteran Mirjana Lucic-Baroni 6-3 2-6 6-0.