Super League leaders St Helens make four changes
Wakefield Trinity welcome back half-back Danny Brough
Winner will join Hull FC and Warrington in semi-finals
St Helens 26-6 Wakefield
Wakefield's Danny Brough kick us off for this second half then.
The injured Kyle Amor is in the BBC studio providing half-time analysis and has said his Saints team-mates know all about Wakefield's comeback wins this season.
Personally, I can't see it.
HT: St Helens 26-6 Wakefield
Image source, SWPix.comBradford caused the shock of the last 16 as they stunned old rivals Leeds while Halifax easily beat Dewsbury
When the full-time hooter goes at the Totally Wicked Stadium in an hour or so St Helens or Wakefield will have joined Hull FC and Warrington in the semi-finals, leaving one spot open.
From 14:30 BST tomorrow we'll be back with more BBC Sport live text commentary of the all-Championship tie between West Yorkshire neighbours Bradford and Halifax at Odsal.
The match is once again live on BBC TV.
One of the clubs will become the first second-tier side to reach the Challenge Cup semi-finals since Hull KR did so in 2006.
St Helens 26-6 Wakefield
Image source, SWpixEmphatic from St Helens - that is why they have lost one match all season and are runaway leaders of Super League.
It looked as though we could have quite a game on our hands in the opening few minutes after Lachlan Coote's try was replied to by a score from Wakefield's teenage debutant Jack Croft. But no.
The boot of Coote put Saints back in front followed by an acrobatic finish from Tommy Makinson and two scores - from Morgan Knowles and Regan Grace - brought about by fantastic length-of-the-field breaks.
Ryan Hampshire's sin-binning for Wakefield has played a part, but even before that Saints were tearing Trinity to shreds. Surely no way back?
We'll see.
St Helens 26-6 Wakefield
Wakefield have a set deep in the Saints half here after Danny Brough's kick forced a touch down behind the St Helens line. They could really do with a try here to have any chance in this one.
Lachlan Coote
Image source, SWpixThe way Saints are playing you'd think they'd want to just keep chasing tries, but they're happy enough to sneak further clear.
Yet another penalty conceded by Wakefield and Lachlan Coote kicks it over.
By the way, Danny Kirmond isn't going to be able to come back on for Wakefield, who are now back to their full compliment of 13 players with the end of Ryan Hampshire's time in the bin.
St Helens 24-6 Wakefield
John Kear
Wales & Bradford Bulls coach
They're not only highly skilled St Helens, they're also very smart indeed.
James Roby noticed it was two on one down the short side and they take full advantage of it.
I gave Regan Grace his international debut for Wales before he'd played for Saints, he scored an 80 metre try and you knew he was going to be something special.
Regan Grace
This has been coming. There were gaps everywhere. Wakefield's defence, stretched like too little butter over a big piece of bread, just couldn't cover everywhere.
Regan Grace is all alone on the short side and plonks the ball down for his 15th, and perhaps easiest, try of the season.
Lachlan Coote can't add the extras.
St Helens have dominated Super League this season and they're showing why this afternoon. They've torn through Wakefield in a matter of minutes.
St Helens 20-6 Wakefield
Saints are cutting through Wakefield's defence at will here.
They have another six minutes against 12 players and I think they're trying to put this match to bed. Butter fingers from Mark Percival with a back-door pass on the right wing prevents a score this time.
Trinity clinging on.
Lachlan Coote
Almost a replica of the attack I just described, honestly.
This time it ends with Ryan Hampshire killing the attack off the ball to stop a certain St Helens try, and he's going to the bin for 10 minutes.
Lachlan Coote adds the two.
St Helens 18-6 Wakefield
John Kear
Wales & Bradford Bulls coach
I love the composure that Saints had kept.
It all came from an error at the other end of the field.
Morgan Knowles should be in the Great Britain squad. He can play long minutes, he's skillful, he can defend and he's very tough.
Morgan Knowles
It gets crazier.
Dom Peyroux seizes on the loose ball after Wakefield throw a poor pass to ground and sprints clear. As he's running out of legs he plays in Kevin Naiqama, but superb defence from Ben Jones-Bishop and a couple of other bodies seem to have stopped the Saints attack.
But they're calm and have won 16 out of 17 games this season for a reason.
They go again, move it to the corner and Morgan Knowles has an easy finish. All that, 30 seconds from their own line.
Lachlan Coote can't quite add the tricky goal.
St Helens 14-6 Wakefield
HUGE error from Jonny Lomax! He drops the kick return and it's a knock-on.
Regan Grace makes things worse by picking the ball up to give away a penalty as he's offside.
Crazy couple of seconds.
St Helens 14-6 Wakefield
Jonathan Davies
Former Wales international on BBC One
That is a magnificent finish from Tommy Makinson, to keep his feet in play was brilliant.
It was the right ball but what a finish.
Tommy Makinson
Craig Kopczak gets overexcited for Wakefield trying to slow the play-the-ball down and Saints can kick their way deep into the Trinity half for a fresh set.
They make use of it, quickly spreading the ball wide for Tommy Makinson to touch down smartly in the right-hand corner. Fabulous acrobatic finish from him as he was running towards the touchline.
That just shows how dangerous it is to give up easy field position.
Lachlan Coote, left-footed, makes no mistake with a superb conversion. Three from three for him.
St Helens 8-6 Wakefield
That breather was possibly needed for both sets of players (and me) - it's truly been 90mph stuff so far.
Saints have a man in the ruck and Wakefield get a penalty so can escape their own half. George King has replaced the injured and concussed Danny Kirmond for the visitors, by the way.
St Helens 8-6 Wakefield
Boos ringing out around the ground as Saints fans are not happy. Danny Kirmond has come straight back on the field after never looking right.
This time, he's got a head knock after a collision while making a tackle and again play is stopped with St Helens on the attack.
The Wakefield second row will surely be replaced this time.
St Helens 8-6 Wakefield
Play back under way after Danny Kirmond has walked gingerly from the field.
Saints can't make use of the last tackle.
St Helens 8-6 Wakefield
Saints are charging towards the line after some great play down the left but Regan Grace runs out of room. Before the last tackle, referee Chris Kendall stops play as Danny Kirmond looks badly hurt for Trinity.
Hopefully he's okay.
St Helens 8-6 Wakefield
John Kear
Wales & Bradford Bulls coach
It was inevitable they'd take the penalty there.
If you win by one you're in the next round, if you win by 100 you're in the next round.
Lachlan Coote
Alex Walmsley had about four blue shirts on him there and they still couldn't stop the big Saints prop. When they did, the Wakefield defenders didn't get off him quickly enough and it's a penalty bang in front.
Lachlan Coote kicks it and Saints are back in front.