Defending Super League champions Wigan Warriors will face St Helens for a place in the grand final after beating Salford Red Devils 18-12.
London Broncos' brave bid to retain their Super League status came up short, while Salford Red Devils claimed third place in the table.
After serving as interim boss at Newcastle Knights, Kristian Woolf will swap the NRL for life in Super League with St Helens.
Jay Pitts' late try means London Broncos remain in with a chance of beating the drop, while also leaving Hull KR in deep trouble.
Super League leaders St Helens were slender victors against Castleford Tigers, while Huddersfield Giants eased relegation fears.
Losers in the Challenge Cup final for two of the last three years, Warrington Wolves turned the tables to beat St Helens and claim glory.
Super League leaders St Helens put in an impressive second-half display as they stormed to a 36-20 victory over Leeds Rhinos at Headingley.
Warrington Wolves offered little resistance against Super League leaders St Helens, who ran out 30-12 winners in Thursday's clash.
Huddersfield Giants were torn apart by struggling Leeds Rhinos at their own ground in Super League on Friday.
Justin Holbrook has led a revival at St Helens and will be charged with delivering fresh impetus when he arrives at the Gold Coast Titans.