O'Sullivan looked near to his fluent best as breaks of 142, 95 and 85 gave him a 4-0 lead at the break.
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Even then, the five-time world champion was not totally confident of victory and went into the second session with a plan to not let the rhythm of the match subside.
"I managed to settle and get a few breaks, to get in at 4-0 at the interval was lovely but even at 4-0 you know its never over with Selby," O'Sullivan told Eurosport.
O'Sullivan: You have to play the game on your terms
"He can make you work for it, graft you out and draw you into the trenches.
"I thought as long as I play one good frame, he can get back to 4-4 but all I need is one good visit and if I get half a chance I could do something.
"But you've got all that going on in the back of your mind but that's why he's such a good champion, he makes you think."
'What a break that was!' - O'Sullivan hits brilliant 142 on way to Selby victory
Although Sullivan has always had the capacity to play as good tactical snooker as anyone, the draining affect it can have on his mental psyche has led him to feel it is not worth engaging in.
"I accept that I'm not going to want to get drawn into a game with defensive side, I can play defensive snooker but when balls are on cushions and the flows been taken out of it, it's probably not going to do me good in the long run," O'Sullivan added.
"So if I get to that stage, I had a plan to get things in the open.
I'd rather lose it quickly than lose a long one."
"You just have to play the game on your terms - set out your stall.
"My mission when I first started to play was to enjoy it, play fluent snooker, so it shouldn't be any different if you're a professional."
Expert analysis: 'He's now favourite to win'
O'Sullivan's good friend and fellow Eurosport pundit Jimmy White was fulsome in his praise of his efforts - especially given his lack of matches before the tournament.
"He today was quality - everyone would tell you, he's now favourite to win the tournament," White told Eurosport.
"And what an achievement, he's hardly played in anything. He had to win a few matches scrappy (in the early rounds), but Ronnie O'Sullivan the way he played has got to be favourite now."