Zhang, who nearly went out in qualifying - her opponent in the last round of qualies, Virginie Razzano, served for the match in the third set - had taken out No.2 seed Simona Halep en route to her first second week at a major, and she continued that giant-killing form Monday night with a 3-6, 6-3, 6-3 win over the No.15-seeded Keys, who looked increasingly hampered by injury during the match.
"It's so tough to play against someone injured," Zhang said. "Maybe two points you're feeling like you cannot play, and then the next three balls, pong, pong, pong, you make two aces, one winner. You don't know what's going to happen. And also last year this happened many times, and I'd almost win the match, but then I'd lose. But this time I really tried to concentrate, and I'm happy I won."
Coming in, Zhang was 0-14 in Grand Slams and had lost 20 straight sets at majors, too - she was considering retirement. She's not thinking of hanging her racquets up now, but she will need a break.
"Right now, after this tournament, I need a long break," the Tianjin native commented.
"I've already played seven matches. Before today I was thinking, 'Okay, today is the final.' When somebody wins six matches at a Grand Slam, it's already the final, right? And at night, it felt like it.
"But I'd already said this year I'll play less tournaments. I don't want to play too much. I want some more time for my life. I can have a holiday, rest, stay with my parents - and practice more too."
Next up for Zhang is Johanna Konta, who will also be playing her first Grand Slam quarterfinal.
Konta and Zhang are tied in their head-to-head, 1-1, though they haven't played since 2013.
? WTA (@WTA) January 25, 2016