London Irish fly-half Chris Malone kicked four penalties to help his team edge past Harlequins 15-9 at the Madejksi Stadium on Saturday.
Rory Clegg landed three for Harlequins but four from Malone and one from Tom Homer allowed Toby Booth's men to prevail.
Malone booted the hosts ahead in the fourth minute but Clegg replied with two three-pointers to put the visitors 6-3 ahead by the 10th minute.
Quins full-back Mike Brown was keen to run the ball at every opportunity but tryscoring chances were scarce in a hard-fought encounter.
Two more penalties from fly-half Malone put Irish 9-6 ahead by the 24th minute but Clegg's third penalty levelled the scores before the break.
The opening ten minutes of the second half mirrored the first half action with neither side able to build up any sustained period of pressure. Homer kicked Irish into the lead with a penalty after 52 minutes and the hosts came nearest to scoring a try when a kick through from Seilala Mapusua narrowly escaped the clutches of Steffon Armitage as the flanker attempted to scoop up the ball to score.
Quins were then dealt a severe blow when Jordan Turner-Hall was sent to the sin-bin for offside at the ruck. Malone was successful with the resulting penalty to give the hosts a 15-9 lead after sixty three minutes.
Following his penalty success, Malone was immediately replaced by Ryan Lamb who within seconds was required to play at scrum-half as Paul Hodgson was sent to the sin-bin.
For London Irish:
Pens: Malone 4, Homer
Pens: Clegg 3
London Irish: 15 Tom Homer, 14 Topsy Ojo, 13 Seilala Mapusua, 12 Dan Bowden, 11 Adam Thompstone, 10 Chris Malone, 9 Paul Hodgson, 8 George Stowers, 7 Steffon Armitage, 6 Jamie Gibson, 5 Bob Casey, 4 Nick Kennedy, 3 Faan Rautenbach, 2 James Buckland, 1 Clarke Dermody (capt).
Replacements: 16 Rob Herring, 17 Max Lahiff, 18 Paulica Ion, 19 Kieran Roche, 20 Richard Thorpe, 21 Jonathan Joseph, 22 Ryan Lamb, 23 Darren Allinson.
Harlequins: 15 Mike Brown, 14 Tom Williams 13 Ugo Monye, 12 Jordan Turner-Hall, 11 Ollie Lindsay-Hague 10 Rory Clegg, 9 Dave Moore, 8 Chris Robshaw (capt), 7 Will Skinner, 6 Maurie Fa'asavalu, 5 Pete Browne, 4 Ollie Kohn, 3 James Johnston,, 2 Joe Gray, 1 Joe Marler.
Replacements: 16 Chris Brooker, 17 Ceri Jones, 18 Mark Lambert, 19 Tomas Vallejos, 20 Chris York, 21 Ben Urdapilleta, 22 Tom Casson, 23 Ross Chisholm.
Referee: Sean Davey