O’Sullivan pinched the first frame but Allen immediately responded in the second before surging on to book his place in the last four at Alexandra Palace.
The Rocket was heavy favourite coming into the tournament after clinching the UK Championship at the end of 2017 before blasting past Marco Fu in the opening round of the Masters.
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However, throughout the clash with Allen, the Northern Irishman believed his opponent was beginning to crumble under the pressure.
Allen insists he went into the match without any fear in his mind and sensed the opportunity to capitalise on O’Sullivan’s errors.
“You can’t go out there and be scared of Ronnie,” he said. “You have to go out there and impact the game yourself and that’s what I’ve always done against him.
“I honestly believe at 4-1 up, he cracked a little bit and I sensed that out there.
“I actually turned one or two down that I could’ve took on. I felt like I didn’t want to give him an easy chance to get his arm going.
“I definitely felt he was feeling the pressure and it’s unusual to say that for someone of O’Sullivan’s stature but it just shows that he’s human.
“I’m very, very chuffed to be in the next round but I’m not going to get carried away, it’s just one match. There’ll be two more tough ones if I’m going to win this tournament.”