The Rhinos looked on course for a comfortable victory at saturated Knowsley Road as they ground out a 19-0 lead with 11 minutes remaining.
But they were shaken by the inspired Jamie Lyon, who scored twice and set up another as Saints hit back in thrilling fashion.
In the end Leeds, after tries from Danny McGuire, Danny Ward, Ali Lauitiiti and a Kevin Sinfield drop goal, just had enough.
The Rhinos now have a fortnight off before the Old Trafford showpiece but Saints� title aspirations are not yet over and they will now play-off against the winners of Saturday�s clash between Bradford and Hull.
Conditions were extremely difficult after a torrential downpour soaked the pitch before kick-off and Saints full-back Paul Wellens was soon in trouble.
Wellens missed the ball as Leeds kicked ahead but the Great Britain international gathered from a lucky deflection after Leeds were unable to take hold.
Yet Saints, missing four senior players through injury and fielding 19-year-old James Roby at scrum-half, created the first two tryscoring opportunities � although they were unable to take either.
Willie Talau first stepped into touch after a menacing Jason Hooper kick and Paul Anderson was then held short after a powerful drive.
Leeds then drew first blood as McGuire capitalised on a strong tackle from Chris McKenna which forced the ball from Lyon�s grasp.
Saints looked to have rode out a spell of Rhinos pressure after defending three successive sets of six but McKenna�s power proved too much for Lyon and McGuire accepted his 22nd try of the season.
Sinfield converted and the champions were close to a second soon after following a high Andrew Dunemann kick but Willie Poching knocked on touching down.
Saints tried to force their way back into the game but were unable to break through fierce Leeds defence despite some excellent possession.
The visitors held firm and extended their lead six minutes before the interval after Rob Burrow forced the Saints defence back.
The ball was quickly recycled and Matt Diskin sent Ward � guilty of two earlier knock-ons � over from close range. Sinfield converted to make it 12-0.
Leeds had Ryan Bailey placed on report after a spear tackle on James Graham and Wellens went close, but the Rhinos went through to the interval unscathed.
Handling errors punctuated the early stages of the second half and neither side were able to make headway.
It was particularly frustrating for Saints, whose gameplan was continually knocked back and hooker Keiron Cunningham was surprisingly withdrawn for Mick Higham.
Leeds maintained their dogged defensive approach and they gained a seemingly decisive grip on the match when Lauitiiti broke from deep for his side�s third try.
The Kiwi second rower blocked a Jon Wilkin grubber kick deep inside his own half and scooped up to scamper 80 metres to the line. Sinfield�s conversion extended the Rhinos� advantage to 18-0.
Lyon went close to clawing four points back for the home side but the ball went to ground in the corner and the video referee denied him the points.
Saints then suffered yet another injury blow as stand-off Hooper left the field with a dislocated left shoulder.
Sinfield gave his side further breathing space when he clipped over a drop goal 13 minutes from time but Saints were not done as Lyon took it upon himself to try to salvage the match.
The centre forced his way over in the corner after 69 minutes, added the touchline conversion himself and broke clear down the right to send Ian Hardman over.
Lyon was unable to convert that time but still had plenty of energy left and got on the end of a slick move to score again as Saints cranked up the momentum. He then narrowed the gap to three points by adding the goal to set up a thrilling finish.
Yet Leeds, in the end, just had enough.
St Helens: Wellens, Hardman, Lyon, Talau, Gardner, Hooper, Roby, Fozzard, Cunningham, P. Anderson, Bennett, Gilmour, Wilkin. Replacements: Higham, Fa'asavalu, V. Anderson, Graham.
Leeds: Mathers, Calderwood, Walker, McKenna, Bai, McGuire, Burrow, Bailey, Dunemann, Ward, Ellis, Poching, Sinfield. Replacements: Lauitiiti, Diskin, Jones-Buchanan, McDermott.
St Helens: (0) 16. Tries: Lyon 2, Hardman. Goals: Lyon 2.
Leeds: (12) 19. Tries: McGuire, Ward, Lauitiiti. Goals: Sinfield 3. Drop Goals: Sinfield.
Referee: Ashley Klein (London) Att: 13,209
- - - - -
Thanks to www.superleague.co.uk for this report.