Saracens unbeaten start to the season was brought to a halt by a brilliant first-half display from Harlequins as they claimed a 17-10 win at the Stoop.
The defending Aviva Premiership Rugby champions had won their last 13 games in all competitions, but found themselves 17 points down by half-time following tries for wing duo Tim Visser and Charlie Walker.
Richard Wigglesworth crossed early in the second half for Saracens as they mounted a fightback of sorts but just as they did a year ago, Quins handed the champions a first loss of the season.
Harlequins lost Joe Marler to injury within a minute but responded with a flying start.
After an early Tim Swiel penalty, they were ten points clear when Visser picked off Alex Lozowski to race 75 metres for a try.
Swiel converted to give Quins the perfect start and put the champions under real pressure for the first time this season.
It was their biggest deficit of the campaign to date but they looked to be coming back into it with one brilliant break from Alex Goode coming to naught after a wayward pass by the full-back.
Instead it was Quins who struck again, piling the pressure on the visitors. Two mauls were illegally stopped to leave Saracens on a final warning, but after they successfully stopped a third, the ball was spread wide and with extra numbers Walker had the simplest of finishes. Swiel converted from out wide to make it 17-0.
Harlequins were in control but almost gifted Saracens a try to get back in it when Mike Brown dithered and Wigglesworth dived for a loose ball on the try-line.
Brown dotted it down just in time, but conceded a five-metre scrum and after a period of sustained pressure Saracens looked certain to score.
But as they were given a final warning for indiscipline, the home side were able to force a knock-on out of Mako Vunipola and clear their lines.
Saracens kept at it, but Harlequins showed their defensive resolve to keep the champions out before the break.
The champions needed a response early in the second half and they got just that with some magic from Mike Ellery.
The winger had bided his time in the first half but when the ball was shifted to the right after the break, he got away, chipped over the covering defence and then produced a perfect offload inside for Richard Wigglesworth to jog under the posts. Lozowski slotted the conversion to cut the deficit to ten.
Things could have got even worse for Quins when Brad Barritt's wide pass looked to have put Saracens in. But some brilliant last-ditch defence, notably from Chris Robshaw, forced the error.
Quins could have extended their lead again soon after but Swiel's long-range penalty attempt came back off the post.
Saracens were proving uncharacteristically error-prone though, with one overthrown lineout and subsequent knock-on symptomatic of their struggles.
And just as Swiel had done earlier, Lozowski saw a penalty effort bounce away off the post as he looked to bring Saracens back within a score just after the hour.
He pulled a second attempt wide not long after but there was a clear sense of urgency about the visitors.
In the first half it was Saracens who had dominated at the set-piece but the tide turned after the break, and a trickle of scrum penalties eased the pressure on the home side.
Still, Saracens weren't going to go down without a fight, and after a couple of successive maul penalties, James Horwill left his partners four minutes from time as he was sent to the sin-bin.
Ben Spencer slotted the resulting penalty to make it a one-score game with three minutes to go.
But there was to be no late drama, with Saracens unable to find a way through and having to settle for a point.