Two first-half tries from full-back Zak Hardaker saw Leeds into the last eight and kept them on course for their first Tetley's Challenge Cup triumph since 1999.
The stand-out fifth-round tie at Headingley lived up to all expectations as the Rhinos showed their class to establish a 20-6 lead before demonstrating their new-found defensive resolve to keep St Helens at bay in a spirited fightback.
Leeds finally clinched a 32-12 victory with two tries from Danny McGuire and Jamie Peacock in the last seven minutes of a quality match to condemn Saints to a third successive defeat.
The game was seen as a clash of the two most exciting sets of outside backs in the English game and it fully justified its billing.
Hardaker took the scoring honours, but England winger Ryan Hall was the biggest thorn in St Helens' side with a series of rampaging runs that earned him the man-of-the-match award.
St Helens, toppled from the Super League summit after back-to-back defeats over Easter, were virtually at full strength, with the return of eight first-choice players, including influential scrum-half Luke Walsh, and he was at the heart of their attacking play.
The tone for the first half was set inside the first carry of the match when Saints prop Alex Walmsley lost the ball to give the home side a perfecting attacking position.
Leeds forced a goal-line drop-out from which Hardaker regained possession to set up the opening try.
Kevin Sinfield combined with his trusty lieutenants Rob Burrow and McGuire to create an overlap on the right, but Hardaker went on his own and stretched out of a two-man tackle to plant the ball over the line for his first try.
Sinfield added the goal to make it 6-0, but Tom Briscoe knocked on from the re-start to help give Saints a foothold in the game.
It needed something special to unlock the best defence in Super League and the visitors produced it on 13 minutes when centre Mark Percival broke out of his own half to send winger Adam Swift over for a try, which was converted from the touchline by Walsh.
Saints had a let-off when McGuire touched down after pouncing on the ball after it came loose from Walsh's grasp, with video referee Ben Thaler deciding the tackle had been complete.
But the momentum stayed with the Rhinos, who gained more reward for their enterprising play four minutes before half-time when Hall's latest powerful burst created the space for Hardaker to weave his way over for a second try. Sinfield added the goal for a 12-6 interval lead.
Sinfield stretched the advantage to eight points with a penalty four minutes into the second half and Leeds were firmly in control after scoring a third try four minutes later.
A break on halfway from ever-dangerous second-rower Carl Ablett set up the position for centre Joel Moon to cross and Sinfield kicked his fourth goal from as many attempts to make it 20-6.
Saints were awarded their first penalty on 51 minutes and that gave them the position to score their second try.
Full-back Jonny Lomax had been well marked, but he unleashed the shackles with a neatly-timed pass to get loose forward Josh Jones over and Walsh kicked his second goal.
That prompted Saints to produce their best spell of the match but they suffered a blow when former Leeds prop Kyle Amor went off with a knee injury.
Leeds also lost lively hooker Burrow with a shoulder injury on 64 minutes, but they soaked up a tremendous amount of pressure before sealing the win.
A break from halfway by centre Kallum Watkins got McGuire over and former England prop Jamie Peacock added the finishing touches with a close-range try just before the end, with Sinfield taking his goal tally to six from as many attempts.