At 45, the Scotsman is the oldest player to reach the last eight at the Crucible since Steve Davis appeared in the quarter-finals, aged 52, in 2010. Before that you have to go back to Ray Reardon in 1985.
McManus’s 13-11 win over Ali Carter saw him join experienced potters Mark Williams, 41, and Marco Fu, 38, in the quarter-finals.
The other confirmed quarter-finalist is world No 1 Mark Selby – something of a pup aged 32 – after he struggled through 13-11 against qualifier Sam Baird.
McManus – who made his Crucible debut in 1991 – is enjoying something of a renaissance, having failed even to qualify for Sheffield between 2007 and 2012.
“I still practise a lot and I work hard at my game,” said McManus.
“Sometimes when I’m out there, I’m looking around and thinking I might never be here again.
“It’s a special place to be, it really is. To be in the quarters with another match to play, I can’t wait.
“I’ll be underdog in my next match and that suits me.
“Two years ago I played Mark Selby in the quarters and I never put on much of a show so hopefully I will this time.
“I’m going to think about that over the next couple of days. I’ll remember what was going on when I was playing that match and see if I can use it as an advantage.”
McManus has reached the quarter-finals five times in 20 visits to the Crucible over the years.
When he trailed 9-7 against Carter on Saturday, the Glasgow potter’s attempt to reach the semi-finals – where he will face either John Higgins or Ricky Walden – for the first time since 1993 looked in doubt.
But McManus – who has climbed back up to 29th in the world rankings – potted the final black to clinch the opening frame, before a 119 break levelled the contest at 9-9.
He picked up the next two frames too, and despite breaks of 71 and 100 from Carter, McManus was on a roll.
“Winning the first frame on the black was pretty big, if I had lost that I would probably not be sitting in the next round right now,” said McManus. “After that I started relaxing into it. My plan was to try to get to 10-10 at the interval so to be 11-9 was a massive bonus.”
It was a sorry end for two-time finalist Carter, who admitted, “I wanted to smash the whole place up and go home” such was his frustration.
“It was a disgusting performance,” said the 36-year-old. “Alan dictated the whole match. I was just powerless to do anything about it, I kept trying to kick into gear but I never really found it.
“It didn’t happen for me. It wasn’t through a lack of trying, I tried my hardest. It’s unusual for me not to perform on the big stage and not to come up with the goods, but everything that could go wrong did go wrong.”
Williams is chasing a third world title, 13 years after he last lifted the Crucible trophy.
The 41-year-old – winner in 2000 and 2003 – beat Michael Holt 13-8 in his second round match. Hong Kong cueman Fu, 38, is now through to the Crucible quarter-finals for the first time since 2006 after his 13-9 win over Anthony McGill.
Monday could see two other 40-year-olds join the last eight, with Higgins and Ronnie O’Sullivan returning to complete their matches.
The pair have nine world titles between them, and Higgins holds a 10-6 lead over Walden.
But O’Sullivan, a five-times winner, has work to do as he trails Barry Hawkins 9-7 in their best-of-25 frames match.
The youngest player left in the tournament is 24-year-old Kyren Wilson, who concludes his match with Mark Allen, 30, on Monday.
Wilson raced into a 7-0 lead, and returns in the afternoon with a healthy 11-5 advantage.
McManus and co have proven that when it comes to the marathon experience of the Crucible, experience is key. That was evident 12 months ago when Stuart Bingham, at the age of 38, used his decades of playing on the circuit to lift snooker’s biggest prize.
One twentysomething is at least guaranteed a quarter-final spot from Ding Junhui’s match with Judd Trump.
Sheffield-based Ding, 29, has an overnight 10-6 lead against Trump, 26, a finalist here in 2011, following their opening two sessions.