Champions Saracens won 'The Big Game' at Twickenham to inflict a first Premiership defeat of the season on leaders Harlequins.
Owen Farrell's boot did the early damage, aided by a try from Quins old boy David Strettle, to open up a 19-6 lead in front of an 82,000 crowd.
Prop Joe Marler crossed after the break but Nick Evans, who had landed two penalties, missed the conversion.
Sarries hung on to close the gap at the top to just five points.
It represented a world record for the highest attendance for a regular season club fixture.
It was also the biggest-ever crowd for any Premiership match, beating the 81,601 who watched the 2009 final at Twickenham between Leicester and London Irish.
Although Saracens did not score another point after the break, they defended resolutely to not only ruin Quins' record-breaking Christmas party and their unbeaten start to the Premiership season, but also to deny them even a losing bonus point.
Mark McCall's men countered Harlequins' natural adventure by slowing the game down and drawing penalties, which Farrell converted at will, the nerveless 20-year-old England prospect slotting three inside the first 13 minutes to put Saracens in the box seat.
Saracens conceded a penalty themselves and, after the first of many shoving matches during a feisty first half, Evans kicked Harlequins on to the scoreboard.
It was a brief respite, Quins adding to their problems when Marler's soft pass inside his own 22, intended for Nick Easter, was intercepted by winger Strettle, who cruised over to score.
Farrell succeeded with the conversion and then slotted a fourth penalty after excellent work from Andy Saull, making his 100th Saracens appearance in place of the injured Jacques Burger.
Harlequins kept hammering away but found no chink in the Saracens armour and eventually had to settle for a second Evans penalty to go in at half-time trailing by 13.
Quins, chasing an 11th straight league win, had to up the intensity after the interval and they did so.
Tom Williams, Matt Hopper and Ugo Monye all threatened before the forwards cranked up the pressure, Marler atoning for his first-half mistake with his third Premiership try.
Evans missed the conversion but Harlequins maintained the pressure. Easter crashed through a hole in the defence and into the Saracens 22, but Maurie Fa'asavalu could not hold on to the offload and the chance went begging.
Harlequins director of rugby Conor O'Shea:
"I am sure Owen Farrell will be a cornerstone for England for a long time. I saw him at Loughborough in a training camp when he was 14. It was an Under-18 camp and he ran the show. He's an outstanding player.
"To give a lead like that to the Premiership champions was going to be difficult to claw back.
"Our guys fought tooth and nail to get back into it and tried to play as much rugby as we could. But, when you look back, you will probably see three opportunities we didn't take. At this level you have to take them."
Saracens director of rugby Mark McCall:
"Andy Saull was absolutely magnificent and, from an England perspective, a lot of our young players really made a good impression. Brad Barritt, who is just getting better and better, Owen Farrell again and Alex Goode who has really come into form in the last month or so. Ben Spencer really helped to close the game out and Mouritz Botha did well when he came on too.
"Charlie Hodgson can't be forgotten either. Some of his long passes today were outstanding. I don't think anyone else in England can pass the ball like him.
"We were in control in the first half but, in the second half, Quins showed why they are top. They played scintillating, off-loading rugby and we had to be at our very, very best to stop them. We've won four in a row and this was against the top side in the league. We put everything into that match and there are a lot of battered bodies now."
Harlequins: Brown; Stegman, Hooper, Turner-Hall, Monye; Evans, Care; Marler, Brooker, Johnston; Vallejos, Robson; Fa'asavalu, Robshaw, Easter.
Replacements: Williams for Stegman (34), Gray for Brooker (58), Fairbrother for Johnston (58).
Not Used: Lambert, Matthews, Wallace, Bolt, Clegg.
Saracens: Goode; Strettle, Farrell, B Barritt, Short; Hodgson, Stringer; Gill, Brits, Nieto; Borthwick, Kruis; Brown, Saull, Joubert.
Replacements: Wyles for Strettle (42), Stevens for Nieto (46), Botha for Kruis (50), Spencer for Stringer (60), Smit for Gill (60), Powell for Farrell (73), Wray for Brown (73), George for Brits (77).
Referee: Wayne Barnes (RFU).