The Chinese veteran first came to prominence in 2009 when she became the lowest ranked player to defeat a reigning World No.1 - Dinara Safina at the China Open - but the 26-year-old had never won a Grand Slam match in 14 previous attempts.
Against Halep, Zhang displayed stunning form from the outset, hitting 31 winners and racing out to a 4-0 lead in the opening set before breaking serve in the 10th game to take the early lead.
Though the Romanian appeared to right the ship early in the second, Zhang weathered the storm and remained aggressive to win the last five games of the match to reach the second round.
"Thank you to everyone for supporting me," a speechless Zhang said in her on-court interview.
"I think today is the best moment."
For the former French Open finalist, the loss marked another abrupt end to an Australian Open campaign, having lost in back-to-back quarterfinals in straight sets. It is also her earliest exit from a hardcourt major tournament since the 2013 Australian Open, where she also lost in the first round.
Zhang will next play Hobart International champion Alizé Cornet; the Frenchwoman has dropped just six games in her last three matches - including a 6-1, 6-2 win over Eugenie Bouchard in the final of Hobart - and was equally ruthless to open her Australian Open against Bojana Jovanovski, winning, 6-1, 6-0.