|Warrington Wolves (8) 18|
|Tries: Gidley, Ratchford, Lineham Goals: Patton 3|
|St Helens (10) 10|
|Try: Lomax Goals: Walsh 3|
Warrington Wolves came from behind to beat St Helens and secure their place in Super League's Grand Final.
If Warrington defeat either Wigan Warriors or Hull FC on 8 October, they will be domestic champions for the first time since 1955.
Stefan Ratchford and Tom Lineham touched down in the second period to send the Wolves to Old Trafford.
Saints had led 10-8 at half-time, with Kurt Gidley's try for the home side cancelled out by Jonny Lomax.
But the visitors were unable to add any further points after the interval and they could feel aggrieved about the award of Lineham's match-clinching try.
Television replays appeared to show that the former Hull winger had knocked on in attempting to ground the ball in the corner.
In reaching the Grand Final, Warrington ended a run of seven successive home defeats by St Helens, a sequence that had stretched back to June 2011.
Wigan host Hull in the second semi-final on Friday.
Ratchford magic sees Wolves to Old Trafford
Both sides had taken a fairly pragmatic approach in the first period, turning down attempts to run sets of six at the opposition and instead electing to take the two points on offer from kicks at goal.
The one real moment of quality before the break came from St Helens, who scored a scintillating breakaway try through Lomax after good work by Jordan Turner and Adam Swift.
Warrington required a spark having conceded a two-point deficit at half-time, and full-back Ratchford provided two excellent creative moments to swing the match in his side's favour.
Ratchford, who appeared in Warrington's two Grand Final defeats in 2012 and 2013, first spotted a gap in the Saints defence to dive over from dummy-half and then fired a pass out wide for Lineham to cross for his third try in two matches.
And, as St Helens attempted to find a way back into the semi-final, Ratchford was rock solid under the high ball and helped to see off any late threats that Saints posed.
'Grand Final win would be something special'
Warrington head coach Tony Smith: "I thought both teams defended really well, we didn't give each other too many opportunities. I thought Saints were fantastic.
"But I just had that feeling that, no matter what happened, they were going to work hard for each other and that's a really big ingredient in rugby league.
"You might not be squeaky clean or just perfect in some areas, but if you work really hard for each other you can cover over a lot of the cracks."
Warrington captain Chris Hill: "[Winning the Grand Final] would be unbelievable and it would go down in history. For me personally being captain, it would be something special and something I'd never forget, but we'll get to that when we're walking up those steps.
"Before that, we've got a big week ahead of us and we're going to prepare well."
St Helens head coach Keiron Cunningham: "I just thought we lacked a little bit of composure in certain areas. That's generally how you win big games - the most composed side for longer periods and whoever kicks the ball best.
"Warrington didn't do anything special, they just did what they always do. I thought we defended a lot of their stuff really well.
"But the biggest thing for me, which is becoming a bit of a blight on the game, is you're looking at the first semi-final and what everybody's talking about is the video referee and refereeing decisions. We just couldn't buy a call tonight.
"You need your fair share of those calls, you do need a bit of luck in the game and it does even itself out over the period of a season, but it seemed like it went a fair bit against us in the wrong game."
Warrington: Ratchford; Lineham, Evans, Atkins, Russell; Gidley, Patton; Hill, Clark, Sims, Wilde, Hughes, Westerman.
Interchanges: T King, G King, Smith, Philbin.
St Helens: Lomax; Owens, Peyroux, Percival, Swift; Turner, Walsh; Amor, Roby, Richards, Greenwood, McCarthy-Scarsbrook, Wilkin.
Interchanges: Walmsley, Vea, Thompson, Knowles.
Referee: Ben Thaler.