The three-time champion edged Stuart Bingham in a tense battle of the former Crucible winners.
Mark Williams is through to the quarter-finals of the World Championship for the ninth time in his career after a 13-11 victory on Friday in Sheffield.
In an entertaining match that ebbed one way and then the other, it looked almost a certainty that the only way it was every going to conclude was in a deciding frame.
There was never more than a single frame in it between Bingham fighting back from 6-3 behind to make it 6-6 until the scores were locked at 11-11 late on.
But Williams, who had failed numerous times to pull away from his opponent, managed to win the last two frames and set up a mouthwatering upcoming clash with either Ronnie O’Sullivan or Ding Junhui.
“I think this morning was the best session out of the three,” Williams told Rob Walker after the completion of the contest.
“When the pressure was right on, I produced my best stuff which is unbelievable really – people can question my game or my pots, but they can never question my bottle when I need it.”
“I practiced hard for the last four months and I’m coming in playing really well in practice. You wouldn’t think it after the first few sessions, but that’s the way it goes as it’s not easy out there.
“I stuck in there and stuck in there, I was throwing frames away and missing easy pots, but that’s just my game now.
“The older I get, the more easy pots I’m going to miss, and it’s just a matter of putting it out of my mind and getting on with the next one when they come.
“Hand on heart, I’d like to play Ronnie because we’ve grown up as juniors together and I haven’t played him here for a while.
“I know he’s going to play out of his skin, and I’m going to have to play out of my skin – even if I do play out of my skin, it might not be good enough.
“Nobody is expecting me to beat him if I play him and all the pressure is on him, so what’s the worst that can happen?
“I’m definitely going to enjoy the experience if I do play him, because how many more times am I going to in theatres like this?”
At 45 Mark Williams will be the oldest Crucible quarter-finalist since Steve Davis in 2010. MJW a little older than Alan McManus was in 2016.
— David Hendon (@davehendon) August 7, 2020
Five-time champion O’Sullivan gets his bid to reach the last eight under way on Friday evening with his opening session against fellow former world number one Ding.
Interestingly, the victory for Williams does manage to extend a remarkable streak in which at least one of the Welshman, O’Sullivan, and John Higgins – the fabled Class of ’92 – has reached the World Championship quarter-finals each year since 1995.
Elsewhere on day eight in Sheffield, reigning champion Judd Trump won six frames on the spin to overturn a 5-3 overnight deficit into a 9-5 advantage against Yan Bingtao.
However, the latter crucially pinched the last two frames of the penultimate session to reduce the deficit heading into their final bout later today.