Unlike the women's draw, the men's side didn't lose a top-10 player until Sunday, when fourth-seeded David Ferrer was upset by Gilles Simon in the third round 6-3, 3-6, 6-1, 6-3. Sixth-seeded Tomas Berdych and No. 7 Grigor Dimitrov also advanced.
Dominic Thiem, 20, made his first Grand Slam round of 16 with a straight-set win over 19th-seeded Feliciano Lopez. Another first was achieved in the women's draw by Peng Shuai, who had never been to a major quarterfinal before beating 14th-seeded Lucie Safarova 6-3, 6-4.
Wozniacki will next face 13th-seeded Sara Errani, who ended the run of qualifier Mirjana Lucic-Baroni with a three-set victory.
As a 19-year-old, Wozniacki made the 2009 U.S. Open final, losing to Kim Clijsters, and reached No. 1 in the world the next year. But she hadn't been back to a Grand Slam quarterfinal since the 2012 Australian Open.
In the meantime, she was best known for getting engaged to star golfer Rory McIlroy — then getting dumped in late May after wedding invitations had gone out.
Wozniacki lost in the first round at the French Open soon thereafter and was upset in the fourth round at Wimbledon. She had been playing much better since, though, winning her first title in nine months at Istanbul then dropping a pair of three-set matches to Williams.
Wozniacki also announced a month ago that she planned to run the New York City Marathon on Nov. 2 for charity, somehow squeezing in training during a busy fall season. Seems to be working out well mentally and physically on the tennis court.
"Kind of clears my head," she said of the extra running. "I feel more free when I go on court."
Wozniacki plans to stick around New York after the U.S. Open to attend Fashion Week — including her pal Williams' show — and get in some running in Central Park. The marathon prep has gone on hiatus the last few days, though.
As Wozniacki deadpanned, "I have a pretty big tournament here that I kind of want to try and win."