O'Sullivan, who has not lost to his final opponent since 2008, was struggling to find his range early on as breaks of 72 and 53 helped Perry claim the opening two frames despite needing several attempts to stagger over the winning line.
A 59 from O'Sullivan in frame three helped him claim his first frame of the final, but Perry - playing in his biggest match in his 26-year career as a professional - headed to the mid-session interval with a 3-1 lead courtesy of a fine 74 in the fourth frame.
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A lovely 115 from Perry in the fifth frame was good enough to carry him into a 4-1 advantage, but O'Sullivan has cut a resilient character this week and dragged himself level at 4-4 by winning the next three frames with some solid if unspectacular snooker.
Perry had the chance to counter attack in the final frame of the session after O'Sullivan had broke down on 55, but missing a black off the spot proved crucial as he developed the final red while missing black enabling the five-times world champion O'Sullivan to clear to black in levelling the final at 4-4.
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It is slightly astonishing that O'Sullivan has won four frames while making only two breaks over 50 so far, a fact that could return to haunt Perry when they return at 7pm on Sunday with another 11 frames possible.
The first man to 10 claims the title and a cheque for £200,000 with O'Sullivan still a warm favourite to win a seventh Masters, a victory that would take him one frame clear of Stephen Hendry as the most successful champion in the invitational event's history.
Desmond Kane at Alexandra Palace
Ronnie O'Sullivan (England) 4-4 Joe Perry (England)
24-79 (72), 16-101 (53), 86-27 (59), 31-90 (74), 0-115 (115), 67-40, 61-21, 81-23 (55)
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