O'Sullivan was a heavy favourite heading into the clash, yet it was Maguire who started the brightest, making a break of 98 to clinch the first frame. But complacency slipped into the world No 5's game, and he went down 6-2 after allowing the Rocket to fire his way into a four-frame lead.
O'Sullivan will face reigning champion Mark Selby in today's final but for Maguire, it was his third semi-final defeat in the Masters after bowing out at the same stage in 2007 and 2010. He was disappointed that Alexandra Palace saw him nowhere near his best after making such a positive start in the opener.
"I started off pretty well and made a good break but it just went to pieces after that, I couldn't control the white ball," the Scot said. "If you don't control the white ball and get good position, then you're going to miss, and that's exactly what I did. The table was beautiful, it was very fast and I just got caught out.
"Six out of the eight frames there I was in the balls, so I've got nobody else to blame but myself. If I can't take chances with the amount I had that's my own fault. I'm trying to take heart, I've reached the semi-finals and have got 50 grand in my bank account so it's not been the worst week of my life."
It was backs against the wall for Maguire for much of the match as Ally Pally roared on O'Sullivan, but Maguire warned the Rocket that he must improve for his clash with Selby after presenting him with opportunities to claw his way back into the match.
"It's disappointing because it was a good atmosphere out there and he handles the crowd phenomenally well; I couldn't do that with the amount of shouts he gets," Maguire said. "But with the chances I got, he'll need to improve because Mark won't be missing as many as I missed. I honestly don't know how that final will go."
Selby had earlier crushed Shaun Murphy 6-1.
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