The O’Sullivan-Selby rivalry has arguably been snooker’s greatest of the 2010s, with the duo facing off in all three Triple Crown finals over the past decade.
The meetings have dried up in recent years, however, with the quarter-final in Glasgow only their second showdown since 2016 and first since 2018 - but it was worth the wait.
'Unbelievable talent Lisowski has come of age' - Selby and White's Big Take
The high-quality affair saw four centuries in the opening five frames, and after going to a decider for an eighth time in 24 meetings, it was Selby who won 5-4.
Selby goes on to face David Gilbert, who beat world champion Judd Trump 5-2 later on Friday evening.
“I always enjoy playing Ronnie. I see it as a challenge, he’s probably the best to have played this game,” Selby told Eurosport.
“You always know you’ll be up against it and you have to play as good as you can possibly play.
“I thought I’ve got nothing to lose. I wasn’t feeling great before the match. I went in as the underdog. Everyone was expecting him to win and I just went out there and enjoyed it.”
O'Sullivan was magnanimous in defeat, saying "that’s what champions are made of” after Selby’s final-frame 63.
“I’m happy with my performance this week. Mark’s a brilliant match player. I think I’ve done alright, I’m happy to be able to compete," he told Eurosport.
“At one stage in that match it went scrappy and I smashed them open. I don’t want to be playing 50, 60-minute frames. I would rather lose one quickly and get on with the next one.
“Mark played well, he played good stuff, good safety and a brilliant break at the end under pressure. That’s what champions are made of.”
Watch: Selby rejects O'Sullivan fist-bump; plays rock, paper, scissors instead
O’Sullivan joked that he hoped Selby would “liven up a bit” going into the match, referencing the latter’s six-minute shot at the Northern Ireland Open.
And 'The Jester from Leicester' - who opted for a humourous rock, paper, scissors exchange with O'Sullivan before the match - duly showed his quality from the off, pouncing on a mistake to take the opening frame with a 120 break.
O’Sullivan then responded with a 123 of his own as the best-of-nine match looked set to be a high-calibre encounter.
A scrappy third frame followed, however, and it was Selby who edged ahead after winning the safety battle.
Two centuries of 113 and 111 either side of the interval saw O’Sullivan then take the lead, only for Selby to hit back and level at 3-3.
O’Sullivan beat Selby at his own game when winning the safety exchange in the seventh, moving within one frame of a semi-final spot.
Selby rallied with a break of 59 to force the decider, and seemingly had it wrapped up in the final frame, only to miss a routine black to end his break on 63.
However, with an advantage of 55, Selby held his nerve after O'Sullivan could not reduce the deficit.
Selby survives safety scare to beat O'Sullivan in deciding frame
In the evening, Trump went into his match having dropped just three frames en route to the quarters, but fell 3-1 behind to an unfazed Gilbert.
After the interval, Trump reduced the deficit with a 101, but Gilbert responded with a magnificent 142 to move within a frame of victory.
At 4-2 down, Trump was trailing 57-28 when he attempted an ambitious plant into the top-right corner.
The red rattled the jaws, and that was Trump’s last shot of the tournament, with Gilbert getting over the line to book a meeting with Selby.
Earlier in the day, Mark Allen raced to a 5-1 victory over Scott Donaldson, having started the match barely 12 hours after his win over Chris Wakelin.
Breaks of 62, 81, 102 and 78 saw Allen through to face Jack Lisowski, who hit a tournament-high 143 in his 5-3 win against Thepchaiya Un-Nooh.
Allen faces Lisowski on Saturday evening, after Selby v Gilbert, although the defending champion - speaking after his win - said he had not been pleased with the scheduling so far this week.
"The schedule this week has been disgraceful when it comes to my matches,” Allen told Eurosport.
“Being defending champ I knew I’d start off on Monday. Then I was 10am Wednesday, 10am Thursday, then last on at night and then first on Friday.
“Some very strange decisions but no one wants to take the blame for it.”