She is only the fourth Chinese woman to make the last eight at a grand slam and her run in Melbourne has been particularly surprising given she had contemplated giving up tennis after failing to progress beyond the first round of 14 successive majors dating back to 2008.
"I've been waiting for one match win (at a slam) for so long, more than eight years," 133rd-ranked Zhang, who struggled to finish off the ailing Keys as nerves took hold, told reporters.
"It's very exciting. I don't want to stop."
Zhang's progress began with the first major upset of the tournament, when she dispatched world number two Simona Halep.
But she was given little chance of reaching the last eight against the hard-hitting American youngster, a semi-finalist last year when she pushed Serena Williams hard.
The 15th-seeded Keys won the opening set in 32 minutes but the 20-year-old needed medical treatment on her left leg after being broken in the third game of the second, an injury she later said was probably a torn adductor muscle.
Keys, who added she also had issues with a stomach muscle in a previous match that flared up again on Monday, found it increasingly difficult to move and the 27-year-old Zhang took advantage to send it to a decider.
Keys looked on the verge of retiring several times in the deciding set but battled on grimly despite being in obvious pain as she reached for wide balls.
"One, I hate retiring; two, you don't want to do that to someone who is trying to get into the quarter-finals," Keys, who was in tears as she limped off court and had to be helped into her media interview by support staff, said.
"I thought maybe I could figure it out and somehow get through then have a day off then try."
The Chinese, who sympathised with Keys after the match, added it had also been a distraction, which undoubtedly contributed to a nervous third set.
"It's so tough to play against someone with an injury," Zhang said. "You could see her pain. You are feeling she can't play then the next three balls 'boom, boom, boom, two aces, one winner' so you don't know what can happen.
"This has happened before, and I lost my concentration. This time I tried to concentrate, so I'm happy I won the match."
The last qualifier to reach the Australian Open quarter-finals was Mexico's Angelica Gavaldon in 1990.
Zhang will now play unseeded Briton Johanna Konta in the last eight with a semi-final place beckoning.
(additional reporting by Ian Ransom; Editing by Martyn Herman)
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