You can't accuse Zhao Xintong of shirking the tricky shots, as he misses a couple of difficult long attempts early in frame 22. He does have quite the cushion in this match, after all.

He isn't punished, and eventually gets the pot to set himself up with what looks like a tasty break chance here.

John Virgo
1979 UK Championship winner on BBC Four

Every time you think there's a little spark coming from Ronnie, he misses an easy one.

Every credit to him though - I know he missed when he was in, but just to keep the faith.

He's pulled another one back, there's still a mountain to climb but at least he's giving it his all.

Ronnie O'Sullivan returns to the table and banks enough easy points to win frame 21.

Not the flowing win he would have wanted, but a frame on the board to settle the nerves and stall Zhao's charge.

That miss came from nowhere for O'Sullivan - he was cueing well, it was a straightforward pot. But he seemed oddly hesitant on that shot; a sign that all is still not well for the seven-time world champion.

The saving grace is that he is not punished as Zhao misses a tricky long red.

Image source, PA Media

Now this is much more like it from O'Sullivan, some clever cueing to keep the break going while moving up and down the table from reds to blue.

He brings up his first 50 break of the evening... but then misses a sitter of a red!

The break stops at 57. Gutting for Ronnie.

Ronnie O'Sullivan makes a strong start to this second half of the session with a red to the middle right, but it will be hard to make a frame-winning break from here with the pink and black both tied up.

But the interval may have come at the perfect time for Ronnie, he looks a little more refreshed.

Well, John is certainly confident. And that would certainly be quite something.

The players are back in the arena, Ronnie O'Sullivan breaking off in frame 21 and he attempts to start the comeback of all comebacks.

Straw poll - if you think Ronnie O'Sullivan can come back and win this match, give this post a thumbs up.

If you think Zhao will see it out, vote thumbs down.

Image source, PA Media

Judd Trump and Mark Williams are locked at eight frames apiece in the other World Championship semi-final in Sheffield.

2019 champion Trump led 7-3 but three-time winner Williams won five of the final six frames this afternoon to ensure it would be all square overnight.

They will resume at 10:00 BST on Saturday and return at 19:00 BST to decide a winner.

Ken Doherty
1997 world champion on BBC Four

It's amazing how it's switched. The pressure is off Zhao, he's playing his usual style - so languid around the table and beautiful to watch.

Had Ronnie potted that red in the previous frame and won it, it might have been a different story.

Now Zhao has switched back on, he's relaxed and what a wonderful break.

John Virgo
1979 UK Championship winner on BBC Four

Image source, PA Media

Ronnie will be a little bit disappointed he didn't reach his goal to win three of the first four - or the bonus of winning all four.

Sharing the frames is not going to be good enough.

BBC presenter Seema Jaswal tells us that was the 100th century of this year's World Championship.

Three century breaks for Zhao in this match, with a new best of 128. O'Sullivan is yet to hit three figures - despite winning two frames tonight, he hasn't hit a half century so far in this session.

If Zhao wins three of the four frames after this interval, he will have eliminated O'Sullivan with a session to spare.

And he's done it with a beautiful century break, his third of the match. Zhao rattles off 128, the highest break of the entire match so far.

The Crucible crowd give a very warm ovation to the man from Xi An. Clearly the majority is backing O'Sullivan, but they know quality snooker when they see it.

BBC Four

Television coverage has now switched to BBC Four - that's where you can watch the rest of tonight's session, as well as on this page by clicking the "Watch and listen" tab.

Winning a frame this evening seems to have settled Zhao right down - he's back at the table here and potting fluently.

Ominous for O'Sullivan.

Shaun Murphy
2005 world champion on BBC Two

We said before the frame that Zhao needed to win a frame very soon and he got some help.

The red Ronnie missed is the type of red - played in that way - you can miss with a new tip. Playing it with side spin, it pushes the cue ball off line and you miss it thick.

We'd seen enough already that Ronnie was looking a little better, but that will be debilitating.

Image source, Getty Images

Frame 20, the final one before the mid-session interval.

The commentary team were discussing that O'Sullivan realistically needs to win tonight's session 6-2 at least. If Zhao wins this one and restores his eight-frame advantage, the task begins to look truly gargantuan for his opponent.

Zhao cleans up, and earns his first frame of the evening.

He needed that, to halt the burgeoning momentum it appeared Ronnie O'Sullivan might have been building.