Following the match, O'Sullivan hinted at retirement before stressing he would return next season.
"I'll always be involved in snooker because it's in my blood but I don't need it," he said.
"But it's like a drug and I know I will miss it, and I'll always think 'What if?'.
"I've had 17 years of trying and hoping and thinking that my game can turn around, so I know I've given it my best and I can't give it any more than I have done."
Asked whether he would play next season, he said: "Of course, I hope so."
You're sitting there thinking that anything can happen but inside you know how it's going to pan out
He revealed he had been resigned to defeat before Selby potted the brown in the final frame.
"You get to know your own engine. You're sitting there thinking that anything can happen but inside you know how it's going to pan out," he added.
Selby admitted he struggled with his game, despite scoring an impressive win.
"For the majority for the match, I didn't play well," he said.
"I struggled. But I always thought I will have every chance - take one frame at the time and you never know."
He added: "All I ever wanted to do is win the world championship.
"I will keep concentrating on my own game and go into the semi-final and give it my all."
For Selby and O'Sullivan, it was a strange day and a strange match.
Both players played poorly on Tuesday and went into Wednesday's second session locked at 4-4.
But the eight frames that followed were some of the best played at this year's championship.
In each of those frames there was a break over 50, with O'Sullivan at his imperious best, making five big scores, including a 117 in the 13th.
This was the Essex player's best snooker of the tournament. Even a disagreement with referee Leo Scullion over a touching ball decision did not throw the three-time winner off his stride.
But, aside from being a great break-builder, Selby is a fighter.
The Leicester player has previously produced comebacks against O'Sullivan - he won 9-8 from 8-5 down against him at the 2008 Welsh Open and then sealed a 10-9 victory from 9-6 behind at January's Wembley Masters.
He won the last two frames of the afternoon session with breaks of 104 and 62 to reduce the deficit to 9-7.
Come the evening, it was back to the sort of gritty snooker which played to Selby's strengths.
The 26-year-old won two of the first three frames of the final session to cut O'Sullivan's lead to 10-9, and would probably have levelled had he not missed a routine black in the next.
Ronnie gets touchy over touching ball
Selby bounced back with a sublime 108 and then took the lead for the first time in the match with a 46 break in the 23rd.
Facing defeat, O'Sullivan then made 36 before breaking down and Selby took advantage following a brief safety scrap, letting out a cry of joy after potting the final brown that left O'Sullivan needing a snooker.
But instead of fighting to stay in the championship, the former champion rose from his chair to shake Selby's hand.
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