2004 UK champion Maguire was never really troubled by former Masters winner Robertson at London's Alexandra Palace as he reached the last four stage of snooker's biggest invitational event for a fourth time.
Maguire saw a 3-0 lead reduced to 3-2 only to win the last three frames fairly comfortably.
The Scotsman admitted he was lucky to be playing after struggling with tonsillitis since his 6-4 win over Joe Perry in the first round.
"I've been in bed for a couple of days with tonsillitis, and was close to pulling out last night," he said.
"It hit me on Wednesday. If it wasn't for my mum and dad getting the antibiotics sent down, I couldn't have played.
"That win is for my mum. I couldn't even speak yesterday.
"I feel a lot better now. The antibiotics have been great."
Maguire has promised to attack O'Sullivan after watching the world champion produce an outstanding display in demolishing Ricky Walden 6-0 earlier in the day.
"I spoke to Ricky afterwards, and he said he couldn't do anything," commented Maguire.
"Ronnie made all his own chances today. If he plays like that, he will beat me. It is as simple as that.
"But tomorrow is a different day. I'll just play.
"Everybody knows Ronnie can play like that. I've played him at Wembley where the crowd were a bit harsh.
"I'm not worried about the Ally Pally... there was some good support there tonight.
"I'm not going to try to tie Ronnie up. I'll be going for my shots. If they go in, I can be a bit dangerous.
"It is so good to be in the semi-finals.
"Apart from the World Championships, it doesn't get any better than this."
O'Sullivan tipped Maguire - then 22 - to dominate snooker after he overcame him on his way to winning the UK title a decade ago, but reaching the semi-finals of the World Championship and Masters is as good as it has got for him since that gilded moment.
"It feels longer than a decade to be honest," said Maguire. "After winning it, I went off the boil to be honest for a couple of years.
"Managed to get it back then went off the boil for a couple of years. So maybe I'm coming back?
"10 years. Yes. I've aged a bit."
Maguire benefited from a sluggish start against Robertson by picking up the opening two frames with both men struggling for any fluency.
The first frame lasted 45 minutes 32 seconds compared to O'Sullivan's 6-0 win over Ricky Walden's match seeing six frames played in 57 minutes 48 seconds.
Maguire showed glimpses of his true form with a 68 and 55 in the third frame, but Robertson responded with 74 in the fourth to register his first frame of the night.
A 41 and 30 from the man from Melbourne helped him to close to within one frame at 3-2 behind, but Maguire quelled his opponent's recovery with a knock of 75 in the sixth frame.
A timely clearance of 40 carried Maguire one frame from the last four after the Scot had produced some sublime safety shots to leave Robertson snookered behind the green in baulk.
With Robertson clearly toiling, Maguire largely had the run of the table in the last frame to progress.
"That is probably the last time I drive myself to a match," said Robertson. "I had no adrenaline, or buzz playing in front of a lot of people.
"It took me over an hour to get here. I think it affected my concentration. It was just an empty performance from me.
"Stephen played solid, but I feel as if I kind of gave him it.
"It was lacklustre, and it would have been great to play Ronnie in the semi-finals."
Mark Selby v Shaun Murphy
Stephen Maguire v Ronnie O'Sullivan