Williams compiled exactly 100 - absolutely impeccable until he missed a red when a much bigger ton looked on.
More importantly, it's sent him into the interval just one frame from the final.
In contrast, Trump must win five in a row.
Williams is powering away with this frame - and a straightforward black has left Trump needing snookers.
A fluffed shot ended terribly for Trump - giving Williams an absolutely gaping chance to build decisively.
And he's doing just that.
One red in particular - off the pack - fizzed into the corner and sent the others opening up like a Catherine wheel.
Trump duly completed a sparkling 104 - only missing a tricky black for the clearance.
And such was his flowing form, his opponent Williams will know more than ever a match turn-around is still possible.
Nonetheless, it's the only one of tonight's four frames that Trump has won.
Trump showed the calm of a world champ with a frame-winning break.
Williams needs snookers now.
But Trump will fancy a century.
Trump maintained his momentum with a stunning blue.
Then came a rather cavalier potting of a red that's left him with no excuse not to wrap this frame up soon.
Another blue took his break past 50 - without reply.
Williams has given Trump two slim opportunities at the start of this 27th frame.
And he deftly took the second one - a red to the middle pocket.
But Trump must realise he can't afford to waste chances like this.
Williams produced possibly his first poor safety shot of the day.
But Trump still required snookers.
The Bristol ace potted two reds and two blacks as he looked for the opportunity to engineer a snooker.
And he pulled it off with his fifth shot.
Williams escaped the snookert - only for Trump to deliver another one, which the Welshman got out of by just nicking the red.
But when Trump returned to pot accidentally, he had little choice but to concede yet another frame, 67-22.
Williams bowed out with a 65 break and decent safety shot.
He needs just one more red to leave Trump needing snookers.
Trump immediately conjured a nasty safety shot - but Williams took on the glimmer of a chance and pocketed it.
He had to settle for that pot - plus a snooker of his own behind the yellow.
Trump got out of it - but it's still looking dicy for him.
Williams duly pouched it via the rest.
Williams has had just a tiny slice of luck - with a helpful kiss - as he continues to add blacks in what looks like a significant break.
Trump has an anxious look as he watches on.
And Williams has been in classic form so far this evening.
Trump got out of a tricky early position (pictured below).
And after he was presented with a routine red on his next visit, he pouched a superb brown.
But incredibly Trump missed an even more straightforward red - to open up the table once more Williams, who is building strongly.
Trump could only make 16.
And after Williams put away a red, we were into safety play.
But the onus was on Trump - as he needed snookers.
And Williams returned to pot a bold red.
He then went for caution - and that safety shot was good enough for Trump to concede the frame.
Williams was cruising to this frame with a 67 break when he missed a straightforward red.
It's given Trump a real chance to turn it around dramatically.
Virtually ever ball looks invitingly placed.
And the World No.1 is picking them off nicely.
Williams looks completely in control of this opening break so far.
A black took him into the 30s - and cannoned the cue ball very helpfully into the reds.
Williams bagged a crisp long red to opne the scoring.
And a blue gave him the chance to loosen up the pack of reds.
He did that reasonably well - so already he could be well set.
The two legends have been raucously welcomed back out to the table at the intimate Crucible theatre.
Trump is breaking off first.
Just a reminder that Mark Williams won four of the last five frames to go into tonight's decisive session 13-11 up on fellow legend Judd Trump.
Williams has been Mr Consistency in this semi-final, also notching two of the three centuries so far.
It's the first to 17 frames for the right to face Ronnie O'Sullivan's conqueror Zhao Xintong.
Zhao Xintong is still in dreamland as he waits to learn who he'll face in the final.
After beating his idol Ronnie O'Sullivan in the semi, the chinese ace said: “I tried to enjoy the Crucible. It’s my first time playing the one-table. I am doing well.
“I just want to play like an exhibition here. I cannot believe I am still here.
“I felt relaxed. I wanted to enjoy the experience. Sometimes the crowd here are for Ronnie and it was nervous for me.
“He deserved that, he is a legend here. I think my family were watching tonight in China.
“I will have a big lunch. Don’t think about snooker.”
Judd Trump might be trailing by two frames - but if he gets up to par he's more than capable of turning it around against Mark Williams.
Trump says he's been upbeat about his game all year - helped by relaxing with his rediscovered love for GOLF.
He said: “I felt confident coming into it. I’ve put in the work.
“I think I played well for the first three games, so I couldn’t really have given myself a better chance.
“In my spare time, I’ve been watching a lot of golf on YouTube. Golf series and different people.
“Because I got back into golf recently, so all my time is spent watching golf.”
Ronnie O'Sullivan's semi-final loss destroyed - or maybe just delayed - his bid for a record eighth world crown.
Here's a look at all the winners over the past 56 years.
BELOW is a list of snooker World Champions by year.
The record is for the modern era, widely considered as dating from the 1968-69 season, when the World Professional Billiards and Snooker Association (WPBSA) took control of the sport.
The first World Championships ran from 1927 - with a break from 1941-45 because of World War II and 1958-63 because of a dispute in the sport.
Joe Davis (15), Fred Davis and John Pulman (both 8) were the most successful players during that period.
Stephen Hendry and Ronnie O'Sullivan share the record for the most titles in the modern era, with seven each.
Most World Titles (modern era)
Judd Trump says he's been sleeping well - but he might not be enjoying the lull before tonight's storm.
The World No. 1 knows Mark Williams needs just four frames to reach the final, starting tomorrow.
Trump said before Williams turned an overnight 8-8 score into a 13-11 lead: “I sleep good here. It’s very, very tiring.
"I normally sleep about nine hours straight through.
“Because it’s so hectic around Sheffield, I don’t really come out of my room much. I try to stay away from everyone.
“Conserve my energy that way. It’s important that I do a little bit of gym work here and there, just to keep sane."
Zhao Xintong will take on Mark Williams or Judd Trump in the final for £500,000.
Here's where all the prize money goes over the whole tournament.
Snooker fans have been denied a refund despite no play on Saturday afternoon at the Crucible.
Zhao Xintong's commanding 17-7 victory over Ronnie O'Sullivan was completed on Friday night with a session to spare.
An exhibition featuring Steve Davis and Dennis Taylor, 40 years on from the latter's victory in their famous 'black-ball final' took place instead.
But the WST was forced to pen a statement after fans railed against the decision not to hand out refunds.
Ronnie O'Sullivan has shared a throwback to his first meeting with Zhao Xintong - hours after the Chinese star knocked him out of the World Championship.
O'Sullivan was outplayed by Zhao at the Crucible and went down 17-7 on Friday night.
The snap showed a young Zhao posing with The Rocket, who captioned the photo: "How it started vs how it’s going."
It was a special effort from Mark Williams this morning.
And BBC pundit John Parrot was in awe of the Welshman's long potting.
He said: "Mark Williams has shown tremendous resolve.
"That's a brilliant session of snooker he has just played there.
"You could see he was 'in the zone' with the balls going into the middle of the pocket.
"It was incredibly difficult for Judd Trump to play against."
