Clinton Toopi went over for one of the Leeds second-half tries
Leeds: (14) 38
Tries: Donald, McGuire, Toopi, Peacock, Webb, Burrow
Goals: Sinfield (6), Burrow
St Helens: (19) 19
Tries: Tyrer, Roby, Wellens
Goals: Sculthorpe (2), Tyrer
Leeds Rhinos put in a powerful second-half performance to sink champions St Helens and go joint top of Super League at Headingley on Saturday.
Leeds trailed 14-19 at the break after Scott Donald and Danny McGuire's tries were eclipsed by scores from Steve Tyrer, James Roby and Paul Wellens.
Saints failed to score after the break as Leeds dominated and took control.
Clinton Toopi, Jamie Peacock, Brent Webb and Rob Burrow all went over to seal the Rhinos win.
The champions were missing Great Britain trio Lee Gilmour, Ade Gardner and Jon Wilkin through injury with Tyrer coming in for Gardner.
Dean McGilvray also got a late call to replace Matt Gidley, who fell ill just before the game.
Tyrer made a dream start by powering over the line for the game's first try following a powerful break from Wellens.
Skipper Paul Sculthorpe added the difficult conversion to make it 6-0 but it was then Leeds' turn to dominate.
Donald grabbed the equalising try on 12 minutes after a superb round-the-man pass by Jamie Jones-Buchanan freed the Australian flier.
Leeds captain Kevin Sinfield added the goal and also converted a solo try from stand-off Danny McGuire on 22 minutes that edged the Rhinos in front.
The visitors were back level four minutes later, though, when a superb offload by Saints substitute Maurie Fa'asavalu gave loose forward Roby a clear run to the line.
Kevin Sinfield edged Leeds back in front 10 minutes before the break with a penalty before Wellens jinked through for his try to put Saints ahead again.
Tyrer added the conversion and Sean Long put over a drop goal in the final act of a thrilling first 40 minutes to open up a five-point gap.
Toopi brought the hosts back into contention and then captain Jamie Peacock scored his first try at Headingley for 11 months.
Webb ran in from halfway and Burrow went over for the final try for the home side, while Sinfield finished with six goals and Burrow one.
St Helens coach Daniel Anderson:
"There are no excuses. We were beaten by the better team. We were outmuscled and outplayed.
"The disappointing aspect was that they scored through the middle of the field which doesn't normally happen but it's something we will fix."
Leeds coach Tony Smith:
"To keep a team like St Helens scoreless in the second half is fairly significant.
"They had a few players out through injuries and sickness on the bus so we are not going to get too carried away with it.
"But there were many good performances, I was pleased with them. We carried out the way we wanted to play all through the game."
Leeds: Webb, Donald, Senior, Gibson, Smith, McGuire, Burrow, Peacock, Diskin, Bailey, Jones-Buchanan, Ellis, Sinfield.
Replacements: Toopi, Thackray, Leuluai, Kirke.
St Helens: Wellens, Tyrer, McGilvray, Talau, Meli, Pryce, Long, Fozzard, Cunningham, Cayless, Bennett, Sculthorpe, Roby. Replacements: Graham, Clough, Fa'asavalu, Moore.
Referee: A Klein (Australia)