Defending Super League champions St Helens kept their nerve to squeeze past Leeds in a thriller and reach their fifth Grand Final.

Twenty-year-old scrum-half Matty Smith, deputising for injured talisman Sean Long, was Saints' hero with a match-winning penalty 13 minutes from the end of a pulsating semi-final at Knowsley Road.

St Helens were forced to defend for long periods but hung on for a precious victory despite being well below their best.

The Rhinos, who had done the double over the Challenge Cup winners in the regular rounds of Super League, will have a second chance to reach Old Trafford when they play the winners of tomorrow's sudden-death semi-final between Hull and Wigan and, on this form, will still fancy their chances of lifting the title.

St Helens, handicapped by the absence of Long and rugged forward Maurie Fa'asavalu, looked the livelier side from the start but they ruined their good work by a host of uncharacteric handling and passing errors.

The visitors had opened the scoring but it was firmly against the run of play.

Leeds right winger Lee Smith, who had narrowly failed to pull off an interception in the opening moments, made no mistake when he latched onto Jon Wilkin's pass meant for Willie Talau and showed a clean pair of heels on an 80-metre sprint to the line.

Rhinos captain Kevin Sinfield added the conversion and had the misfortune to shave the outside of an upright with a penalty after Saints prop Nick Fozzard was pulled up for illegal use of his arm in the tackle.

The try did not deter the champions from their free-flowing football, however, and they duly equalised on 15 minutes when stand-off Leon Pryce cut back inside and wrong-footed the Leeds defence with his jinking run that took him all the way to the line.

Smith kicked the conversion and edged his side in front after 21 minutes with a penalty after he had been held down following a searing break.

But the champions had a let-off when a grubber kick from Leeds hooker Matt Diskin rebounded off a post into the grateful arms of full-back Paul Wellens with a trio of attacking players ready to pounce.

Leeds drew level 10 minutes into the second half when St Helens centre Matthew Gidley was punished for a high tackle on Ian Kirke and Sinfield put over a 40-metre penalty.

When Saints were awarded a penalty in a similar position, they chose to run the ball but their enterprise floundered when Gidley lost the ball on the first tackle.

Leeds, grateful for the let-off, almost scored at the other end when Saints passing broke down and it needed Paul Wellens to come to the rescue after substitute Jordan Tansey hacked the loose ball forward to the line.

Saints followers needed no reminding that their team failed to score in the second half of both previous encounters with Leeds this season and the possibility of extra-time loomed as the game entered the final quarter.

Leeds scrum-half Rob Burrow attempted to break the deadlock on 65 minutes with a drop goal but his kick struck an upright and bounced back into play.

Smith was accurate enough with a second penalty after Jamie Peacock had been pulled up for a high tackle on Fozzard to edge his side back in front but Saints were indebted to a last-ditch tackle from the bloodied Gidley to deny Brent Webb in full flight for the line.