Harlequins became the first side to defeat Saracens this season with a 29-23 win at The Stoop, as Rhys Gill was sent off.
A crowd of close to 15,000 welcomed the players on to the field, and there was drama during the early stages of the game as Quins' James Horwill was shown a yellow card following a collision with George Kruis.
Saracens made the numerical advantage count and they took the lead with the game's first try on 11 minutes, scored by Neil de Kock following excellent work at the scrum.
Farrell added the conversion, but he was wide with a penalty shortly after, and Quins quickly found a response through Rob Buchanan's try, scored after Danny Care hit Charlie Matthews' line-out ball at pace.
The conversion was missed, preserving Sarries lead, and they went further in front when Billy Vunipola forced his way over from the back of the scrum.
Farrell converted to make it 5-14, though Nick Evans narrowed the gap to six points with a penalty for Quins.
It remained that way until the final minutes of the first half, when the hosts hit the front thanks to a try from Jack Clifford, following good work from Evans, and Evans' own conversion, which made it 15-14 at the break.
Saracens were back in front within five minutes of the restart thanks to Farrell's penalty goal and he added another on 53 minutes to make it 15-20.
Saracens then had a try ruled out for obstruction, before substitute Ben Botica narrowed the gap to two points with a penalty for Quins.
Two crucial moments in the match followed as the TMO first ruled a red card for Sarries sub Rhys Gill for a dangerous tackle, and the hosts capitalised on their advantage, after Farrell had nudged the visitors further in front with another penalty.
A quick-fire penalty double from Botica put Quins a single point in front with less than ten minutes remaining, before Horwill crashed over to seal a dramatic win for the home side.
Tries: Buchanan, Clifford, Horwill
Pens: Evans, Botica 3
Yellow Card: Horwill
Tries: De Kock, B Vunipola
Cons: Farrell 2
Pens: Farrell 3
Red Card: Gill
Harlequins: 15 Mike Brown, 14 Marland Yarde, 13 George Lowe, 12 Jamie Roberts, 11 Ross Chisholm, 10 Nick Evans, 9 Danny Care (c), 8 Jack Clifford, 7 Luke Wallace, 6 Chris Robshaw, 5 Charlie Matthews, 4 James Horwill, 3 Adam Jones, 2 Rob Buchanan, 1 Joe Marler.
Replacements: 16 Dave Ward, 17 Seb Adeniran-Olule, 18 Joe Gray, 19 George Merrick, 20 Harry Sloan, 21 Karl Dickson, 22 Ben Botica, 23 Charlie Walker.
Saracens: 15 Alex Goode, 14 Chris Ashton, 13 Marcelo Bosch, 12 Duncan Taylor, 11 Mike Ellery, 10 Owen Farrell, 9 Neil de Kock, 8 Billy Vunipola, 7 Will Fraser, 6 Jackson Wray, 5 George Kruis, 4 Jim Hamilton, 3 Petrus Du Plessis, 2 Jamie George (c), 1 Mako Vunipola.
Replacements: 16 Jared Saunders, 17 Rhys Gill, 18 Titi Lamositele, 19 Maro Itoje, 20 Jacques Burger, 21 Richard Wigglesworth, 22 Charlie Hodgson, 23 Nick Tompkins.
Referee: Craig Maxwell-Keys
Assistant Referees: Ashley Rowden, Simon McConnell
TMO: Keith Lewis