James Roby scored two second-half tries as St Helens started the Super 8s in style by ending Super League leaders Castleford Tigers' 12-game winning run with a 26-12 away victory.
The Tigers ended the regular season 10 points clear at the top of the table, but Saints consigned Daryl Powell's men to their first home loss in just over a year on Thursday.
Roby inspired Saints to a fourth consecutive victory which moved them up to third, crossing for two of their four tries and playing a huge part in a superb defensive display from Justin Holbrook's men.
Saints led 8-0 at half-time after Zeb Taia burst through for the opening try, which Mark Percival converted five minutes before adding a penalty.
Adam Milner went under the posts to get the Tigers on the board, but Roby responded with a second four-pointer for the visitors and Jonny Lomax got on the end of a kick through from the fit-again Matty Smith to give Saints breathing space.
Pervical added the extras on both occasions and, although Joel Monaghan finished brilliantly in the corner for Cas' second try, the influential Roby ended any hopes of a late fightback from the hosts by going over again four minutes from time.