|Betfred Super League|
|Castleford (0) 12|
|Tries: Milner, Monaghan Goals: Gale 2|
|St Helens (8) 26|
|Tries: Roby 2, Taia, Lomax Goals: Percival 5|
St Helens became the first side to win this year at Super League leaders Castleford as they sealed victory in the opening game of the Super 8s.
Saints led 8-0 at half-time after Zeb Taia crossed over on 19 minutes.
Adam Milner's try reduced the arrears but James Roby and Jonny Lomax stretched Saints' lead to 14 points.
A spectacular Joel Monaghan try was just a consolation for Cas and Roby's second try secured St Helens only their third away win of the season.
Castleford, who lead by 10 points at the top of the table, were beaten in front of their own fans for the first time since Warrington won 42-26 in July last year.
The visitors went ahead when Taia burst past Zak Hardaker's challenge to go over and Percival added the extras, before converting a penalty five minutes later.
Kyle Amor had a score disallowed for a knock-on to deny St Helens a second try, while the hosts had one chalked off for offside.
Matty Smith's drop-goal attempt for the hosts fell well short on the stroke of half-time, but St Helens went to the break with an eight-point lead.
Milner jinked in under the posts from close range and Luke Gale's conversion reduced the deficit to two points, but St Helens surged back when Lomax's short pass set up Roby for his fifth Super League try in the past six matches - and then Lomax pounced on a bobbling ball from Smith's low kick.
Monaghan's one-handed dive in the right-hand corner gave the hosts a ray of hope, before Roby powered over from close range to secure a convincing victory for the Saints, who move third in Super League.
Castleford boss Daryl Powell:
"I don't think there was much between the teams, but they defended pretty well and there were some key turning points in the game.
"It was pretty tight leading up to half-time, but I thought Saints played really well and I expected them to.
"We haven't had this feeling for a fair while, but I think it will probably help us moving down the track. Our effort was fantastic and the intensity of the game was great, but they were the better side."
St Helens boss Justin Holbrook:
"It was always going to be a tough game, we knew that and we expected it, so to come here and get the win, I couldn't be happier.
"No matter how much homework you do, Castleford are still a very dangerous side and it was up to our boys to continually make the right decisions in defence.
"We did that and it was really pleasing, plus James Roby is as a good a nine as I've seen. He's not just tough and creative, he's also fit and he's a great guy."
Castleford: Hardaker, Monaghan, Minikin, Shenton, Hitchcox, Roberts, Gale, Millington, McShane, Sene-Lefao, Foster, McMeeken, Massey.
Replacements: O. Holmes, Milner, Cook, Patrick.
St Helens: Lomax, Makinson, Morgan, Percival, Grace, Fages, Richardson, McCarthy-Scarsbrook, Roby, Thompson, Taia, Peyroux, Wilkin.
Replacements: Smith, Walmsley, Amor, Knowles.
Referee: Robert Hicks (RFL).