Harlequins defied the absence of their England stars to stun Saracens 23-12 after coming alive in the second half of their Gallagher Premiership clash at Tottenham Hotspur Stadium.
Marcus Smith, Chandler Cunningham-South and Fin Baxter were rested following the completion of the Six Nations last weekend, while Cadan Murley and Danny Care were injured, but their understudies rose to the occasion magnificently.
Trailing 12-0 until the arrival of the final quarter, they crossed through centre Ben Waghorn and then crept ahead through three penalties from replacement fly-half Jamie Benson.
Will Porter put the icing on Harlequins' first away win over their London rivals since 2012 by racing over in the 79th minute as the underdogs gave their play-off hopes a significant boost.
Saracens were poor in front of a crowd of 54,414 and the misery of their defeat was made worse by the fact it came despite fielding a full complement of England stars, apart from Ben Earl who had been ruled out by an infected knee wound.
Initially Saracens appeared to have emerged from the Six Nations break in better shape by crossing after just five minutes when a quickly taken free-kick by Jamie George was followed by a strong carry from Theo McFarland, who dived over.
The move had begun when Alex Dombrandt threw an intercept pass and, while Quins almost cracked for a second time soon after, they regrouped well and spent the rest of the first quarter testing the home defence without success.
Saracens' ferocious breakdown work forced a timely turnover near their own posts, but they could not escape their half for any length of time as a ragged match continued to produce numerous unforced errors.
Quins were controlling territory and possession but points proved elusive, with one promising raid down the right wing hinting at a try, only to be foiled by alert defending.
For a second successive time Saracens' scrum was driven backwards and the visitors' attack was becoming bolder and more accurate, but still without reward.
To show them how it was done, the six-time Premiership champions struck on their next visit to the 22, with a sublime pass by Alex Goode out of the tackle collected by Fergus Burke and Tobias Elliot finishing in the corner.
Saracens should have been over for a third try when Maro Itoje sent Juan Martin Gonzalez charging through a gap in midfield, but the move broke down when the Argentinian was penalised for not releasing under the posts.
Andy Onyeama-Christie came on for his first appearance since October having recovered from ankle surgery and the Scotland flanker had to slot in at inside centre after Nick Tompkins departed for an HIA.
Quins were finally off the mark in the 60th minute when Waghorn finished an athletic break by Oscar Beard and the score brought fresh belief to their play.
Over went three successive penalties from Benton - the second a monster kick - and for the first time Harlequins were in front.
They finished with their tails up as Porter went over late on.
