A penalty try and a score from Danny Care gave Harlequins a gritty 26-15 win over London Irish at Twickenham.
Irish struggled at the scrum all afternoon and eventually left with nothing after a series of punishing set-pieces from the home pack secured their sixth consecutive victory in all competitions.
Harlequins were far from their expansive best in tricky conditions but were clinical when it mattered, Care's score opening up a lead that the Exiles struggled to claw back throughout the second-half.
Quins opened the scoring thanks to penalty at the scrum, Nick Evans converting to give the home side a 3-0 lead.
Ian Humphreys responded with Irish's first points of the afternoon, with the former Ulster fly-half making a couple of clever kicks to the corner in the opening twenty minutes to keep Harlequins in their half.
Both defences held firm throughout the opening quarter, but a second penalty at the scrum to Harlequins gave Evans a simple opportunity in front of the posts, which he converted to give the hosts a 6-3 lead.
Irish hit back from the restart after Maurie Fa'asavalu was pinged for going in at the side, Humphreys converting.
Quins' dominance in the scrum led to a turnover, with Evans releasing Tom Williams on an inside side ball to set up an attack within Irish's 22. A penalty at the breakdown gave Evans a chance to add more points with the boot, but his kick went wide to the left.
Evans then found the corner from a penalty to put Harlequins on the attack before the interval, a driving maul leaving the hosts just five metres out. Care burst left and fed Evans on a line for the posts, but the ball was knocked forward in the tackle by Tagicakibau.
A box kick from Darren Allinson put Ugo Monye under pressure, with the England wing conceding the penalty at the breakdown to give Humphreys a shot at goal, his attempt falling wide to the left.
Irish regained possession from the restart winning another long-range penalty, but Humphreys came up short once more as Irish passed up crucial points.
A third Harlequins penalty at the scrum presented Evans with a shot from from 41 metres out to retain the lead, which he failed to convert to leave the score at 6-6 by half-time.
Evans found his range at the beginning of the second half after Irish were penalised for not rolling away, giving Harlequins a three-point lead.
Irish's scrum responded to their first half troubles with a penalty of their own, finding touch in the 22 before winning another penalty from the resulting maul which Humphreys converted.
A break from Ugo Monye resulted in a Harlequins scrum within the London Irish 22 - Quins winning a penalty after multiple reset scrums and kicking for the corner.
Irish drove the maul back but Care's quick dart across the field found Evans who was tackled short of the line. The England scrum-half then picked up the ball from the ruck and burrowed over, referee Greg Garner confirming with the TMO that the try had been scored.
Humphreys responded with a penalty for the Irish but the Exiles were penalised immediately from the restart, Evans extending the gap back to seven points and leaving the score at 19-12.
The kicking duel continued after another Irish attack won them a penalty within Harlequins 22, Humphreys grabbing his fifth shot at goal of the afternoon to cut the gap to four.
The champions continued to make unforced errors going into the final ten minutes as they failed to kill off the result - before a break from centre Matt Hopper took them behind the Irish defensive line and into the 22.
Quins continued to hammer the Irish line before new signing Pat Phibbs was sin-binned for the Exiles, the hosts winning a penalty and opting for a scrum.
After two collapsed scrums broke down under the strain of Harlequins attack, referee Garner ran under the posts to award the penalty try to seal the victory and deny Irish a losing bonus point.
Try: Care, Penalty
Pens: Evans 4
Con: Evans 2
For London Irish:
Pens: Humphreys 5
Yellow Card: Phibbs
Harlequins: 15 Mike Brown, 14 Tom Williams, 13 Matt Hopper, 12 Tom Casson, 11 Ugo Monye, 10 Nick Evans, 9 Danny Care, 8 Nick Easter, 7 Chris Robshaw (c), 6 Maurie Fa'asavalu, 5 George Robson, 4 Olly Kohn, 3 James Johnston, 2 Joe Gray, 1 Joe Marler.
Replacements: 16 Rob Buchanan, 17 Mark Lambert, 18 Will Collier, 19 Charlie Matthews, 20 Tom Guest, 21 Karl Dickson, 22 Ben Botica, 23 George Lowe.
London Irish: 15 Tom Homer, 14 Topsy Ojo, 13 Jonathan Joseph, 12 Sailosi Tagicakibau, 11 Marland Yarde, 10 Ian Humphreys, 9 Darren Allinson ,8 Chris Hala'ufia, 7 Jebb Sinclair, 6 Declan Danaher (c), 5 Matt Garvey, 4 Bryn Evans, 3 Halani Aulika, 2 David Paice, 1 Max Lahiff.
Replacements: 16 Scott Lawson, 17.Jerry Yanuyanutawa, 18 Leo Halavatau, 19 George Skivington, 20 Jamie Gibson, 21 Guy Armitage, 22 Steven Shingler, 23 Pat Phibbs.
by Ben Coles