Challenge Cup: Iestyn Harris' semi-final guide with St Helens, Catalans, Warrington and Leeds

Four stars, four teams, just two places up for grabs in the Challenge Cup final

Four teams, two games, one stadium and a huge carrot of a trip to Wembley are in store at Bolton's University of Bolton Stadium on Sunday in the Challenge Cup semi-finals.

The ingredients are mouth-watering, with free-flowing St Helens facing a resurgent Catalans Dragons while title-chasing Warrington Wolves keep up their hopes of multiple success against a Leeds Rhinos side desperate to keep their season alive.

Former Rhinos and Bradford Bulls playmaker Iestyn Harris, himself a Challenge Cup winner in 1999, assesses both of the ties which will be live this Sunday from 12:15 BST on BBC One.

Saints to slay the Dragons?

Josh Drinkwater and Ben Barba have been the catalysts for their respective clubs' form

Catalans have been terrific, really terrific. They've found not just their form, but also their style of play. I think they'll really fancy their chances in that semi-final.

Head coach Steve McNamara has added a bit of stillness and calm to them that I don't think we've seen from the Catalans side before.

If St Helens are going to win it they've got to play with that free-flowing style of rugby that they've been so renowned for this season. You just know that their back three are going to be good. You just know that when they're on form and get in those quick play the balls, they are so difficult to defend against.

Where Catalans need to be really good is in those first three plays where they've got to deal with Ben Barba, Tommy Makinson and Regan Grace.

If they deal with that backfield, the presence and size of the Catalans forwards will then come to fruition. If they don't deal with those first three plays, they could be backpedalling for much of the game.

Since signing for Catalans, Josh Drinkwater has given them some confidence. It's allowed Greg Bird to move back to the back of the scrum where he's most effective - although the talismanic Australian is suspended. All of a sudden then, the outside backs are starting to play well because they have started to gel together.

And when the big forwards now get those off-loads, there is a bit of structure behind them now.

Leeds keep season alive or Warrington double dream goes on?

Prolific wingers Josh Charnley and Ryan Hall will find it hard to hide on Sunday, both will be high on their opponents' tip sheets

You say Warrington are on form, but what you never do is write off Leeds in any semi-final. Leeds just know how to win silverware. They've done it over the last 10 years.

Kevin Sinfield is now back at the club, he's changed and tweaked a few things over the last two weeks and given a bit of confidence back.

Warrington will be disappointed off the back of the recent close defeat by St Helens.

I'm going to tip Leeds. I just think they know how to win big games. On paper you'd say Warrington, but in a big semi-final I'm going to tip the Rhinos. I think Leeds can go into semi-finals now and it's just another game so they can relax.

I don't think Warrington are quite there yet. When the Wire go into a big game, they're thinking "we have to do something because this is a big game". You could see that in the Saints game.

When it got into the last 15 or 20 minutes, they were looking for that big play constantly when actually 'playing the percentages' gets you into those winning positions.

Iestyn Harris was speaking to BBC Sport's rugby league correspondent Dave Woods.