Betfred Super League
St Helens (8) 18
Tries: Cross, Dagnall, Sailor Goals: Feldt 3
Leeds Rhinos (0) 4
Tries: Lumb
St Helens took full advantage of a sloppy Leeds Rhinos performance to claim a deserved Super League victory.
Kyle Feldt kicked two penalty goals for the home side in the opening 25 minutes before Deon Cross extended Saints' advantage by darting over for the game's first try on the stroke of half-time.
Leeds improved after the break and brought themselves back into the contest when Riley Lumb went over in the corner for his 10th try of the season.
However, a string of handling errors in stifling heat limited the Rhinos' momentum, and tries from Owen Dagnall and Tristan Sailor guaranteed the points for an injury-hit Saints side.
Leeds stay third in the table on 20 points after their first defeat in six, while St Helens are fifth, two points behind.
Rhinos won 17-4 when the sides met last month, but they were below par from the off in Lancashire.
Feldt's two penalty goals came after Harry Newman committed a high tackle and James Bentley strayed offside, and those infringements were reflective of an error-strewn first period from Brad Arthur's side.
Leeds' one real opportunity in the first half came via a Ryan Hall interception, but the veteran winger was halted by a superb tackle by player-of-the-match Harry Robertson.
And when Leeds coughed up possession in their own 20 not long after, Sailor released Cross, whose clever show-and-go deceived Hall and provided a clear path to the line.
Half-time offered much-needed respite for Rhinos, who came out with more intensity at the start of the second half.
They came back into the contest after a magnificent Lachie Miller break - the Australian full-back evading several tackles before moving the ball out wide, where Lumb dotted down.
Jarrod O'Connor went close for Leeds with a darting effort, and as the game opened up, the momentum swung back and forth before St Helens delivered a telling blow.
Off the back of a scrum, deft hands from Jonny Lomax and Sailor put Dagnall into space. He stepped off his left foot to score his second try in two starts and Feldt converted to make it a 10-point lead.
Leeds - who completed just 69% of their sets and made 16 errors - never truly looked like bridging that gap.
Sailor made sure for Saints when he burst through a gap in the final stages to make it 18-4.
St Helens: Sailor, Feldt, Robertson, Cross, Dagnall, Lomax, Mbye, Walmsley, Clark, Lees, Wingfield, Whitley, Knowles.
Interchanges: Paasi, Delaney, Stephens, Burns.
Leeds Rhinos: Miller, Hall, Newman, Handley, Lumb, Connor, Croft, Palasia, O'Connor, Oledski, McDonnell, Gannon, Watkins.
Interchanges: Lisone, Holroyd, Bentley, Smith
Referee: Jack Smith.