Michael Emons
BBC Sport at the Crucible Theatre

Five seeds have gone already and another one finds himself in huge trouble too. Mark Williams, seeded sixth, started the day with a 5-4 lead, but a disastrous session sees him lose all four frames in a row.

Williams could not wait to get out of the arena at the mid-session interval, sitting on one of the steps by the exit as Si Jiahui completed the fourth of his four frames on Tuesday.

The Welshman, a three-time champion here, was showing his frustration in that final frame of the session, punching his cue after missing a red and jokingly offering his cue to someone in the crowd.

It's been a tournament of shocks so far and another one could well be brewing here. Those in the media centre are sensing a huge story on day four.

John Parrott
1991 world champion on BBC Two

Mark Williams has missed one or two but he's being put under pressure.

The young man is playing absolutely superbly, the balls he's knocking in are going in the centre of the pockets, the positional play has been great.

The only surprise is when he misses one.

Stephen Hendry
Seven-time world champion on BBC Two

He really has come out here this afternoon and stamped his authority on this match.

It’s the business end of the match against one of the most in-form players in the world at the moment.

You’d think there would be a bit of nerves but there’s none of it – he’s just come out and dominated this session.

Si Jiahui broke down on 67 but Mark Williams needs snookers.

He's 58 behind with 59 on. There's a chance to get back in the frame after potting a couple of reds.

But he misses a pot to middle, bangs his cue with his fist and offers it to a member of the audience.

Williams then goes and takes a seat near the exit, so he's closer to the dressing room for the upcoming interval as Si finishes off with some exhibition shots.

It's proving a tough afternoon so far if your name is Williams.

Robbie is now three down against Mark Allen, and it's not really happening for the Englishman right now.

Image source, PA Media

Dennis Taylor
1985 world champion on BBC Two

Ronnie O'Sullivan might have a rival here in switching hands.

When Si switches hands, he looks much the same as he does when he's playing with his right hand.

Two brilliant reds into the middle pocket from Mark Williams - a slow roll and an ultra fine cut - get him in first in frame 13.

But he's unlucky to leave only a tricky red along the baulk cushion after crashing into the pack of reds off the black.

He misses - and Si Jiahui rolls out the old Ronnie O'Sullivan party trick of switching to his wrong hand, rather than using the rest, to pot that same red. He's in again.

Shaun Murphy
2005 world champion on BBC Two

Mark hasn’t put the chances away and Si is full of confidence now.

As I know myself from last year, once Si gets on a roll he’s very hard to stop.

In the BBC studio, Hazel Irvine just mentioned that Si Jiahui won 10 straight frames in his final qualifying round against compatriot Wu Yize to win 10-4 from 4-0 down.

The Chinese won 11 successive frames in his semi-final here against Luca Brecel last year before crumbling in the face of the Belgian's fightback.

Point is, he can rack up frames - and quickly.

Stephen Hendry
Seven-time world champion on BBC Two

Big frame coming up. Si has got momentum which is so important in multi-session games. Mark Williams needs to stop that quickly.

Five seeds out already - including Mark Selby, defending champion Luca Brecel and Ali Carter.

Is Mark Williams about to become the sixth? Si Jiahui takes a third frame on the spin with a break of 61. One frame to go before the mid-session interval.

Image source, Getty Images

Si Jiahui runs out of position on a red so the break ends on 28 but he's back in soon after as Mark Williams leaves a snickable red near the corner pocket.

The table is open and he's 50-odd already. 7-5 incoming.

A missed yellow from Robbie Williams allowed Mark Allen in and the Northern Irishman is now two ahead.

Not fazed at all, Shaun, is he?

A chance at a long red to start frame 12 and Si Jiahui dispatches it like it was a formality.

Another chance for a sizeable contribution for the 21-year-old.

Shaun Murphy
2005 world champion on BBC Two

He doesn't look bothered out there, Si. He doesn't seem to be fazed or feel any nerves, he's just chilled out.

That's one of the biggest attributes - playing out there likes it means nothing when it means everything. It's a real good recipe - you can learn everything else.

Dennis Taylor
1985 world champion on BBC Two

It will be a very relieved Si Jiahui because if he had lost that frame after that incredibly unlucky in-off, it might have knocked his confidence.

But Mark Williams can't take advantage - the final red stays out off the jaw off the middle pocket, much to the Welshman's dismay.

Think he thought it drifted a bit.

Would have been harsh on Si to lose that frame after the unfortunate in-off - and he cleans up to take a 6-5 lead.

The Chinese is in first again in frame 11 and compiles a lovely 63 break, only to break down in sight of the winning line.

Mark Williams, though, cannot punish a missed red, making just seven points after running out of position.

However, he'll get another chance as Si is so unlucky to go in off with the cue ball after knocking in a lovely plant.

Stephen Hendry
Seven-time world champion on BBC Two

Mark Williams has got to make sure he doesn't let Si get up a head of steam and get too confident, and start enjoying himself out there.

Because he does look so comfortable playing here at the Crucible.

He looks like he belongs here, in among the best players.

One of the big stories emanating from this year's tournament so far is the debate over the future of the Crucible Theatre as the host venue for the World Championship.

The event has been staged in Sheffield for almost 50 years since 1977 - but its deal to host the tournament runs out in 2027 when it reaches that half-century milestone.

Iran's Hossein Vafaei this week said the venue "smells" and the tournament should move elsewhere, something John Higgins thinks is "inevitable".

However, Shaun Murphy branded Vafaei's comments "almost sacrilege" while Alan McManus told Eurosport: "You come here to play snooker, you don't come here to smell roses and flowers and all that stuff. There's nothing wrong with the Crucible."

What do you think? Should the Crucible remain snooker's iconic home? Or must the sport move with the times and relocate its showpiece event? Get in touch via the usual methods.

Image source, Getty Images