Maguire was far from his best initially as the five-time world champion rattled off three half-century breaks on his way to a 4-0 lead at the interval.
But the 36-year-old rallied to win four of the next five frames before the Rocket produced a break of 63 to edge over the line and into today’s final with a 6-4 victory.
Maguire, the former champion, admits he believed O’Sullivan was rocking as he brought the score back to 5-4 before finally losing.
He said: “I’d have liked to have taken it to 5-5 because you could see he was starting to rock a little bit. Nobody likes it when someone is coming back at them.”
Yet Maguire knows that gifting O’Sullivan a four-frame head start was too big a gap to bridge.
He said: “It [going 4-0 down] definitely cost me. I just didn’t settle until the interval at 4-0. Then it’s too late. I put up a bit of a fight but I had lost in those first four frames sadly.
“I hate losing. I lost that before the interval which I’m gutted about but even at the end I fancied doing the job.
“I wasn’t looking at the scoreboard because I knew if I was in I could clear up. It’s a pity it was too late.”
Nobody would have blamed Maguire for being unhappy at his early showing but the Scot revealed he wasn’t too downhearted going into the mid-session interval.
He said: “I was speaking to my dad and my manager John Rea at the break. I usually smash up the changing room but I felt OK.
“I wasn’t blaming my own game. By the time I relaxed it was too late.”
O’Sullivan will now head into his seventh UK Championship final but the Rocket believes he’s lucky to have made it all the way.
“I’ve got lucky, everyone’s seemed to struggle against me and you’ve seen the snooker yourself, I’ve definitely not been producing what I have been producing over the last couple of months,” he admitted. “You’ve got to tough it out sometimes, there’s no player in the game that can play bad and win apart from Mark Selby, he’s the only guy.”
O’Sullivan, who is a five-time winner of the UK Championship and is now into his seventh final at the ranking event, added: “Stephen is a quality player and I expected some kind of resistance.”