And afterwards the contest descended into controversy as Selby labelled O'Sullivan "disrespectful"
for some of the shots he produced, most notably when he had a smash at the cue ball
to gift Selby a frame, before reeling off three in a row to win, including 276 unanswered points.
The epic encounter followed another unforgettable match between Kyren Wilson and Anthony McGill which remarkably also went the distance, Wilson prevailing after one of the most stunning frames ever seen in snooker.
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In another deciding frame in the evening game, the pair exchanged safety shots before O'Sullivan potted the first red.
Mark Selby on Ronnie O'Sullivan: 'I felt it was disrespectful to me and the game'
He went on to build a break of 65 but needing just a red and then a colour to win the match and secure his place in the final, he missed a distance pot.
Selby put in a break of 34, finishing with a fine defensive shot, leaving the cue ball tucked behind the green.
But O'Sullivan responded well, tipping it off the cushion to gently knock into the final red.
After exchanging safety shots, the Rocket finally got that elusive red, moving him into the final against Kyren Wilson.
"When he's on form, he can be unplayable," said Eurosport expert Jimmy White afterwards, reflecting on a spell in the final three frames in which O'Sullivan put together a run of 276 points.
"You've got to feel for Mark Selby there - he's done nothing wrong...but Ronnie was just incredible."
'If I'm snookered, I'm going to smash them' - Ronnie explains controversial moment
O'Sullivan had begun the evening well, winning the first two frames to make it four on the spin in the match overall.
That meant the scores were level for the first time since 5-5.
But the Rocket was prone to errors, most notably with a bizarrely wayward effort at the pink in the closing stages of the 30th frame.
With the scores 53-47 in Selby's favour, he had presented O’Sullivan with the shot, leaving the pink just behind the black.
The Rocket missed his first effort entirely, adding six points to Selby's total, and with the second he simply hit the cue ball with massive velocity.
'An absolutely ridiculous shot' - Ronnie O'Sullivan smashes cue ball
O'Sullivan compiled a very quick century break in the next frame, taking eight minutes to put in 138 and clear the table, and stayed just as focused to take the next as well, forcing a decider.
World Seniors Championship
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