Former New Zealand international Zane Tetevano was sent off as Leeds relinquished their hold on the Betfred Challenge Cup despite a late rally by their 12 men.
The 30-year-old forward, who was playing only his third match for the Rhinos, was at the heart of a tremendous backs-to-the-wall effort by the patched-up holders, who restricted Super League champions St Helens to an 8-6 half-time lead in their third-round tie at the Totally Wicked Stadium.
But Tetevano’s dismissal four minutes into the second half turned the game the way of Saints, who went on to secure a 26-18 victory that takes them a step closer to ending a 13-year wait for cup success.
St Helens made an ideal start, with winger Tom Makinson forcing his way past full-back Jack Broadbent for the opening try after only two minutes but Mikolaj Oledzki crashed through to bring the visitors level after 10 minutes.
Saints always looked more likely to score but lacked composure at crucial times with the only other points of the first half coming from a penalty by Makinson.
Trailing 8-6 at the break, the holders were dealt a huge blow when Tetevano was shown a red card for an ugly challenge on scrum-half Theo Fages.
Makinson kicked the subsequent penalty and Saints made the extra man count by working left winger Regan Grace over for their second try two minutes later.
As the home side piled on the pressure, Makinson raced clear down the wing only to put a foot on the line and James Roby had another try disallowed by the video referee before Jonny Lomax’s cut-out pass enabled Grace to score his second try.
Makinson’s fourth goal made it 20-6 and looked to have sealed the win but a darting Brad Dwyer had other ideas as the replacement hooker forced his way over for two tries in six minutes from dummy half.
Rhys Martin added both conversions to cut the deficit to two points but Saints finally clinched the win when Makinson went over for their fourth try 90 seconds from the end.