The American extended her sorry losing streak since winning at Flushing Meadows in September to seven matches against the 34th-ranked Chinese number two, a surprise quarter-finalist in Melbourne two years ago.
Stephens came back from a long-term injury to record her US Open victory but has struggled since with form and fitness.
Things looked back on track for the woman known as "Sloane Ranger" as she raced to the first set 6-2 in 38 minutes.
But Zhang would not lie down and gradually got herself on top, winning the second set on a tiebreak for the loss of only two points.
She then scooted away to victory in an hour and 58 minutes over an increasingly frustrated Stephens.
"She's a tough player," said Zhang, who has happy memories of Margaret Court Arena -- Zhang reached the last eight there after coming through three qualifying rounds two years ago.
"I'm so happy to play on this court and play a great player," she added. And the Chinese star had a warning for the rest of the field. "I know how hard I have worked so I'm ready for every match, every player."
She will face either of the unseeded pair Denisa Allertova of the Czech Republic or France's Pauline Parmentier in the second round....