|Challenge Cup semi-final|
|Leeds Rhinos (12) 24|
|Tries: Hardaker, Moon, Peacock, Watkins Goals: Sinfield 4|
|St Helens (4) 14|
|Tries: Percival 2, Savelio Goal: Walsh|
Challenge Cup holders Leeds Rhinos beat St Helens at Warrington's Halliwell Jones Stadium to reach the final at Wembley for a fifth time in six years.
In a last-four tie between Super League's two leading sides, Zak Hardaker and Joel Moon put Leeds ahead.
A superb effort by Rhinos veteran Jamie Peacock made it 18-4, but two tries by Mark Percival kept Saints in touch.
However, Kallum Watkins crossed late on to ensure Leeds will play Warrington or Hull KR in the final on 29 August.
The Yorkshire club's Wembley opponents will be decided on Saturday (kick-off 12:45 BST, live on BBC One).
Leeds have now won all three of their fixtures against St Helens in 2015, with their two convincing Super League successes built on dominant starts.
The Rhinos, who are one point clear of Saints at the top of the league table, began the semi-final in similar fashion and took an early lead when Moon's mazy run gave full-back Hardaker the chance to dart through a gap in Saints' defence.
Kevin Sinfield kicked the resulting goal and then a penalty to give Leeds an eight-point lead, before Rob Burrow's break from inside his own half created the space for centre Moon to cruise over unchallenged.
Percival's effort in the corner just before half-time, superbly created by full-back Adam Quinlan, gave Saints some hope heading into the second period, but it was nearly extinguished by a third score by Leeds shortly after the break.
Former England captain Peacock, in his final season as a player, belied his 37 years by showing a turn of speed and neat footwork to race over the line from 30 metres.
St Helens, who have not reached the Challenge Cup final since they last won the trophy in 2008, responded brilliantly to close the gap to four points.
Young forward Andre Savelio scored from close range and then set up centre Percival for his second of the game to increase the tension before a vociferous crowd in excess of 11,000.
But England centre Watkins secured victory and a return to Wembley for Leeds, sidestepping the St Helens defence to touch down for the decisive try.
Their win was soured, however, by a serious knee injury suffered by Jamie Jones-Buchanan, which is likely to keep the Leeds forward out for the remainder of the season.
Leeds head coach Brian McDermott:
"I know it's a big call but the first 20 minutes was the best I've seen us play. We got a grip of the game there and we didn't let the grip go.
"Saints threw everything at us in the second half and we had to tough it out, so all in all it was a good mix.
"We were holding on for dear life at times. I wouldn't say we were comfortable, we had to draw on some toughness and resilience to see it through."
St Helens head coach Keiron Cunningham:
"I was disappointed with the start. We probably gave Leeds a little bit too much respect.
"I could see the players were gassed but I knew, if we were within a score or two at half-time, we would be in with a shot.
"We came out really well in the second half and at 18-14, the game was on, but we made the misread on Watkins, which was down to poor attention to detail, and it ended up costing us."
Leeds Rhinos: Hardaker; Briscoe, Watkins, Moon, Hall; McGuire, Sinfield; Cuthbertson, Aiton, Peacock, Ablett, Delaney, Jones-Buchanan.
Replacements: Keinhorst, Garbutt, Burrow, Leuluai.
St Helens: Quinlan; Dawson, Percival, Turner, Swift; Burns, Walsh; Amor, Roby, Walmsley, McCarthy-Scarsbrook, Greenwood, Flanagan.
Replacements: Jones, Masoe, Vea, Savelio.
Referee: Ben Thaler