Five penalties from Ben Botica and two from Louis Grimoldby were enough for Harlequins to see off Bath at the Stoop.
In a try-less encounter, Stephen Donald scored three penalties and Tom Heathcote one of his own as Bath kept in touch, but the visitors slipped further away after Grimoldby's two kicks stretched the lead and handed Quins their second victory in the competition.
Meanwhile a late penalty from Danny Cipriani was enough to give Sale a narrow victory over Saracens at Salford City Stadium.
There were two tries each for both sides as Mark Cueto and Fraser McKenzie crossed for the hosts, with Kameli Ratavou and Joel Tomkins doing the same for the visitors.
The result was only Sale's third win this season in all competitions.
Harlequins 21-10 Bath
Sale 25-23 Saracens