Zhao began confidently by taking the opening frame, yet the match shifted sharply when Zhang responded with total clearances of 127 and 129.
Though the world champion levelled at 2-2 before the interval, Zhang already looked the steadier of the two, converting his early chances with precision while Zhao struggled to establish any sustained rhythm.
The turning point arrived in frame six when Zhao, attempting to recover control after a string of earlier misses, overcut a black that should have levelled the match at 3-3. Zhang capitalised instantly, moving 4-2 ahead and tightening his grip on the tactical exchanges.
Zhao's long-potting, usually a strength, remained unreliable, and Zhang continued to dictate the pace with disciplined safety and assured break-building.
He moved to 5-2 with another controlled visit, and Zhao's final attempt at survival ended when he drastically overcut a pink during a clearance.
Zhang required only a simple pot to complete a landmark victory and secure a quarter-final meeting with either Shaun Murphy or John Higgins.
Ding downs Donaldson
On the other table, Ding Junhui advanced to the last eight with a 6-5 win over Scott Donaldson in a match that developed more tension than the early stages suggested.
Ding built a 3-0 advantage with steady scoring, later extending it to 4-2.
However, Donaldson, who overturned a 5-0 deficit against Stuart Bingham in qualifying, once again showcased his resilience.
He punished Ding's lapses with composed visits and eventually drew level at 5-5, forcing a deciding frame.
The decider initially opened in Donaldson's favour, with a fine long red preceding an unfortunate collision with the pink that disrupted his position.
Ding then produced a superb pot to the top left corner and used the opportunity to construct the decisive break.
A clean split of the pack created the layout he needed, and he closed with a century to secure his passage into the quarter-finals.
Thursday's evening session sees John Higgins facing Shaun Murphy and Barry Hawkins taking on Elliot Slessor.