Saracens climbed back into the top four of the Aviva Premiership Rugby table with a hard-fought victory, built largely on the back of a splendid performance by their teenage fly-half Owen Farrell.
The 19-year-old son of coach and former Great Britain rugby league and England rugby union international Andy kicked four penalties and converted tries by David Strettle and Schalk Brits.
Postponements elsewhere meant Saracens could recapture their top four place at the expense of their old London rivals. while Harlequins have their hearts set on achieving Heineken Cup qualification.
It was the visitors who made the more positive start with a fourth-minute try by George Lowe. A big hit on Brits and a late tackle on Farrell caused confusion during a Saracens attack up the right, the ball squirted behind Jacques Burger and into the path of wing Lowe who had an unopposed run home. Nick Evans' attempted conversion went wide.
But once they regrouped, Saracens got on the scoreboard with a Farrell penalty in the 11th minute after Chris Robshaw failed to roll away after making a tackle.
With Harlequins now on the back foot, Saracens created a fine try in the 18th minute. A couple of mini-drives up the left gave Chris Wyles the ball to feed Farrell, who spotted former Harlequin Strettle thundering in off his wing. Strettle took the pass at pace and side-stepped Will Skinner to score behind the posts.
Farrell added the conversion, then kicked a penalty in the 27th minute, although the scoreline did not reflect the home team's superiority at this stage in the first half.
An exciting battle came to a halt in the 29th minute when former England centre Ollie Smith required treatment for a leg injury that required a stretcher to end his involvement. Evans was the sharper when the game resumed, forcing errors to earn a penalty which he fired over in the 31st minute.
However, Saracens effected a turnover two minutes later, sparking a fluent move involving Strettle and Burger that then became a driving maul which Harlequins could not withstand.
When the bodies untangled, last up, clutching the ball, was prolific hooker Brits, scorer of his fifth try of the season. Farrell completed the conversion. Evans reduced the arrears with a 38th-minute penalty to allow Harlequins to change ends still in touch, mainly due to their defiant defence.
Saracens took the initiative from the restart when Mike Brown knocked on in front of his posts, a mistake that led to a succession of re-set scrums. But while no scores resulted, the pressure was still on the visitors 15 minutes in.
Tempers frayed as the half unfolded with referee Andrew Small forced to warn a group of scrappers that any further handbags would see him produce the yellow card.
Saracens added to their tally in the 58th minutes when Farrell dispatched an impressive penalty to punish Harlequins for side-entry at a ruck, but five minutes later, Evans rifled one over from about 40 metres to restore the interval points margin.
Farrell, watched by his father, Great Britain rugby league legend and former England rugby union centre Andy, nudged Saracens back to 12 points clear with a close-range penalty in the 66th minute after Ceri Jones was caught offside.
Harlequins then launched their best attacking spell of the game, but Nick Easter ended a sustained surge by hanging onto the ball after being tackled, letting Saracens off the hook.