Zhou was one frame away from knocking out O'Sullivan but for the second day in a row, The Rocket emerged victorious in a decider.
The world number one will take on Hossein Vafaei in the semi-finals.
O'Sullivan put his stamp on proceedings quickly by racing into a 3-0 lead before Zhou got on the board in the fourth frame.
O'Sullivan would then open up his lead back three which looked to set up a comfortable-looking rest of the match, but Zhou would rally.
The Chinese would cut into O'Sullivan's lead and then go ahead himself for the first time on the day after winning four consecutive frames.
That put him within one frame of a huge win, but the experienced O'Sullivan regained to book his place in the final four and become the first player to win 100 matches at the UK Championship.
In the day's other match, the aforementioned Vafaei saw off Zhang Anda 6-4.
Both players exchanged the first eight frames before the Iranian seized control of the match.