Judd Trump faces an uphill task as he aims to reach a fourth World Championship final.

Here's what happened in the previous three.

Mark Williams is closing in on a fifth World Championship final at the age of 50.

He's scaled the Crucible mountain three times before.

Steve Davis
Six-time world champion on BBC Two

It's all about timing the run. It looks like Mark has timed it exceptionally well.

He's clinical, he's also methodical without being too slow. And of course he strokes the ball so well. When he's got the cue ball all around the black spot area, he's not putting much effort in at all, he knows exactly where the cue ball is going.

It's a joy to watch - the only person who doesn't like watching it is Judd.

Stephen Hendry
Seven-time world champion on BBC Two

It's what the greatest players do - they run to the winning line, they don't shy away from it. It doesn't seem to matter what Judd does, it doesn't alter Mark's path to get to the final. Under pressure, he's producing better and better snooker the more pressure there is.

Dennis Taylor
1985 world champion on BBC Two

Typical Mark Williams. His opponent knocked in a century break in the previous frame, put the pressure on him, and this is how he responds.

A break of exactly 100, and Mark Williams goes into the interval totally in command.

When the players return, Williams needs just one more frame to earn a place in the final.

Judd Trump's century in the last frame was the 101st of this tournament. Both men have made two 100+ breaks in this semi-final.

Mark Williams is rapidly closing on number 102, and his third of the match.

Judd Trump sits in his seat, arms folded with the cue resting between his legs.

He knows he is going to have to come back after the interval and win every frame.

Image source, PA Media

Stand down, Mark Williams has potted a yellow. No 147 here.

But he has so many options after that wonderful positional shot to scatter the reds, and is potting smoothly in this break.

Mark Williams scatters the reds expertly, and is wonderfully placed to win this frame.

Indeed, there is a chance for a 147...

Stephen Hendry
Seven-time world champion on BBC Two

Judd is a savage scorer. It's the second time he's got more than 100 centuries in a season. The bedrock of his success is his scoring and his long-potting.

His long-potting has suffered in this semi-final and I think that's why he's behind in this match.

He's an inspirational player - he needs inspiration from something, and maybe he'll get it from that one visit.

Ooohs from the crowd, as Judd Trump gets his safety shot all wrong and the cue ball bounces off the lip of the middle right pocket.

Mark Williams could be in for a big score here.

From Judd Trump's perspective, at least this means he has forced this match beyond the mid-session interval tonight.

After losing that last frame, Mark Williams briefly left the arena. He has now returned, and breaks in the 28th frame.

Ken Doherty
1997 world champion on BBC Two

A big last frame to come before the mid-session interval. Judd Trump has to keep in touch with Mark Williams.

A possible six frames left to play tonight and Judd is going to need to win five of them.

Trump cleans up and records his 106th century of the season. ONE HUNDRED AND SIX. Remarkable.

More pertinently, he closes the gap to three frames.

A really good response from Judd Trump, keeping Mark Williams in his seat after the Welshman missed that long pot early in the frame.

Trump is eyeing a second century break of the match here.

An absolutely wonderful shot from Judd Trump - very straight on the blue, he plays it with immense power and side to send the cue ball off three cushions and back down the table to the reds.

Shot of the day so far, as Trump moves on to a half-century break.

Judd Trump is under serious pressure - but he is the world number one who has made more than 100 centuries this season. If anyone can respond, he can.

He's building a solid break in this frame, including some tricky shots which required the rest.

Image source, PA Media

Steve Davis
Six-time world champion on BBC Two

There's been an astonishing turnaround in this match. At 7-3 in front, Judd looked in really great shape, Mark looked a little apprehensive and missed a couple of easy shots.

The turnaround has been remarkable.

Mark Williams has missed a couple of long reds early in this frame, and after the second one leaves a red into the middle left which Judd Trump snaffles.

Trump, you would think, needs to win this frame.