Leeds (12) 18
Tries: Ablett, Hall 2 Goals: Sinfield 3
St Helens (10) 10
Tries: Gilmour, Flannery Goals: Eastmond
Leed's Keith Senior was shown a yellow card late on
Two tries from winger Ryan Hall pushed Leeds Rhinos to the brink of winning the Super League Leaders Shield.
Leeds now go into the final game of the season two points clear of St Helens with a far superior goal difference.
Saints took the lead through Lee Gilmour with Kyle Eastmond adding the extras before Carl Ablett hit back.
Hall put Leeds ahead before half-time but Chris Flannery's bustling try brought Saints back into the game until Hall's second try secured the points.
With both teams locked on the same number of points with two rounds of the regular Super League season to play it was always highly likely that the winner of this clash would take the League Leaders' Shield.
The Shield, which has been won by Saints for the past four years, will actually be decided next Friday when Saints face Catalans Dragons at home while Leeds are at Salford. But the intimidating capacity home crowd made this clash feel like a Grand Final.
And there was no shortage of commitment from either side with the early exchanges typically fierce. Leeds had England forward Jamie Jones-Buchanan placed on report for a crunching tackle on Saints front rower James Graham, while St Helens' Leon Pryce was sin-binned for a blatant professional foul.
The visitors took the lead despite being reduced to 12 men when Graham slipped the ball out of the tackle for on-rushing second rower Gilmour to power over for the game's opening try, the 100th try of the 31-year-old's career.
Eastmond added the conversion to put the visitors 6-0 up and, despite an overwhelming share of possession, it took Leeds until the 25th minute to draw level.
The Saints defence was superb but intense Rhinos pressure led to second rower Ablett eventually finding a chink after livewire scrum-half Rob Burrow had been held just short of the line.
And Leeds' pressure told again just before half-time when the Rhinos switched play from one side of the field to the other before veteran centre Keith Senior got winger Hall over at the corner. Kevin Sinfield's second goal opened up a six-point lead.
Flannery's bustling try in the second half brought Saints back into the game but Eastmond's awkward kick from the corner got caught in the wind to leave the Rhinos two points ahead.
The two evenly matched sides continued to cancel each other out with resolute defensive work until Leeds broke through with Hall crossing in the corner for his 28th try of the season. However, with Sinfield missing the extras the six-point gap set up a tense finish.
A Sinfield penalty eased the tension with eight minutes to play and all but assured the Rhinos the League Leaders Shield.
The game boiled over late on as Rhinos' Lee Smith smashed into Gilmore before both players traded blows and Senior was then shown a yellow card after clashing with Gilmore as both sides teetered on the brink of losing control.
Rhinos coach Brian McClennan:
"There was loads of passion and intensity and some outstanding defence from both sides.
"It was a step towards the finals.
"It was a big classic match, pretty much what you are going to get when you get to the semis.
"Defence won it for us. We had to defend our line a few times. We were strong in that area and worked hard for one another.
"Our attack wasn't as good as it has been in the last few week but that has a lot to do with how St Helens defended."
St Helens coach Mick Potter:
"It was a very good match.
"You could see the passion in both teams and that was reflected in some of the other stuff that went on.
"They get the two points but we can take some positives out of it.
"We didn't do enough to win game, though. We made too many errors, we weren't good enough."
Leeds: Webb, Donald, Smith, Hall, McGuire, Burrow, Leuluai, Diskin, Peacock, Jones-Buchanan, Ablett, Sinfield, Senior.
Replacements: Lauitiiti, Burgess, Kirke, Bailey.
St Helens: Meli, Pryce, Long, Gidley, Wellens, Gardner, Wilkin, Eastmond, Graham, Cunningham, Puletua, Gilmour, Flannery.
Replacements: Hargreaves, Roby, Clough, Fa'asavalu.