Michael Robertson and Tommy Lee scored second half tries as London Broncos earned a first home win over St Helens since 2002 with a 21-14 success at the Stoop.
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The win, the Broncos' first home victory this season, ended a run of 13 straight victories in the capital for the Saints.
London rang the changes ahead of kick-off, Warrington loanee James Mendeika made a first start at centre, with Dan Sarginson and Olsi Krasniqi returning to the starting line-up.
St Helens, smarting from their Challenge Cup exit last weekend, gave a debut to Luke Thompson, with Nathan Ashe slotting in at stand-off and Adam Swift replacing the suspended Francis Meli.
The visitors were also missing the injured Mark Flanagan, as well as the banned pair Jon Wilkin and Ade Gardner, both forced to sit out the contest after offences in the defeat at Hull KR.
The Londoners were the ones asking all the questions early on, forcing Saints on to the back foot thanks to the promptings of skipper Craig Gower.
But the Broncos were unable to convert that pressure into points, finding the St Helens defence a tough nut to crack.
Having withstood those assaults, the visitors showed how it should be done with the opening score on 17 minutes, Anthony Laffranchi powering over from close range after the hosts had knocked on close to their own line.
And within minutes the Saints had struck again, Swift squeezing in at the corner after an Ashe kick had ricocheted kindly for Nathan Brown's men.
Swift suffered a leg injury in the act of scoring though, forcing him out of the action.
London were far from out of the contest though, and hit back with their first try on 25 minutes, Chris Bailey crashing over after a short pass from Gower.
It was proving an even contest all round, with the action ebbing and flowing.
Luke Dorn thought that he had brought London level when he skidded over minutes before the break, only to be adjudged as knocking on.
Then Stuart Howarth went close for Saints, burrowing for the line but coming up just short on the final tackle.
In the final action of the half, the visitors threatened again, only for Sia Soliola to be stopped in his tracks by a great hit from Bailey.
The Broncos were probably unfortunate to be behind at the break, but put that right seconds after the restart, Robertson taking a looping pass from Dorn to canter in at the corner.
The hosts had their tails up and stretched further clear on 48 minutes, Lee touching down after Sarginson had intercepted and raced clear.
St Helens huffed and puffed after that, but were getting precious little change out of the London defence, despite the best efforts of Ashe and Paul Wellens.
But with 12 minutes to go it was game back on, as seconds after Kieran Dixon was held just short for the Broncos, Mark Percival streaked clear for the away side, allowing Wellens to cross in the corner.
The Saints threw everything at the hosts in the closing moments, but were unable to find a decisive score, Michael Witt booting a penalty and drop-goal in the final two minutes. Earlier he had converted all three tries.