Harlequins overtook their London rivals Wasps in the table on Saturday thanks to recording a 17-10 victory on a boggy Twickenham Stoop pitch.
Seen by a sell-out 14,282 crowd and England attack coach Brian Smith, Quins ran in three first-half tries and then hung on desperately just four days before Martin Johnson names his squad for the Six Nations Championship.
Joe Marler, the 20-year-old prop called up to the England training camp as injury cover during the autumn internationals, full-back Mike Brown and current England number eight Nick Easter supplied the scores which enabled Quins to lead 17-7 at the interval.
But, playing with very little possession in the second half, they had to dig in to deny a Wasps side who had won seven of their last eight matches in all competitions.
Marler, a star of the England Under-20 team, out-shone Wasps' experienced Tim Payne - capped 22 times by England before losing his place to fit-again Andrew Sheridan in the autumn.
The Wasps pack, without England's Simon Shaw and Joe Worsley, who were both rested, competed with Quins in a tough forward battle, with Wales number eight Andy Powell always looking to make the hard yards and veteran former France captain Serge Betsen working tirelessly.
Wasps dominated the opening stages with England centre Riki Flutey, playing at fly-half after missing the autumn Tests with a calf injury, Powell and Richard Birkett, returning in the back row for his first match since October, all prominent.
They pounded away but a well-drilled Quins defence soaked up all the pressure before the home side took the lead against the run of play.
In Harlequins' first significant attack, winger Tom Williams was stopped just short after influential former New Zealand fly-half Nick Evans hoisted a kick to the corner.
But, moving the ball quickly from right to left, Quins created space for Marler to charge over and dive in near the flag.
Wasps hit back to take the lead five minutes later when full-back Mark van Gisbergen timed his run perfectly to link up as the extra man in the three-quarter line and provide the final pass which sent right-wing Richard Haughton racing in for a try converted by Van Gisbergen.
Harlequins responded with a wave of attacks which ended when scrum-half Danny Care, looking to regain his England starting spot from Ben Youngs, moved the ball quickly from the base of a ruck to put Brown over in the left corner.
Evans converted to increase their lead to 12-7 and Harlequins went further ahead three minutes before half-time when Easter drove in through a crowd of players as Wasps defended desperately on their own line.
The visitors made a quadruple substitution nine minutes into the second half, with scrum-half Joe Simpson and regular goal-kicker Dave Walder, both rested from the starting line-up, replacing Nic Berry and Flutey while props Payne and Zak Taulafo gave way to Charlie Beech and Ben Broster.
The changes gave Wasps a new impetus and, even though Walder missed two penalties, he confidently struck the third in the 61st minute to reduce the deficit to seven points.
With Wasps enjoying a new lease of life, Harlequins were again forced into a rearguard action but their well-drilled defence held out as the over-anxious visitors, left with a losing bonus point, lacked the final attacking precision to make the breakthrough in a hectic finish.
Tries: Marler, Brown, Easter
Con: Van Gisbergen
Harlequins: 15 Mike Brown 14 Tom Williams 13 George Lowe 12 Jordan Turner-Hall 11 Ugo Monye 10 Nick Evans 9 Danny Care 8 Nick Easter 7 Chris Robshaw (capt) 6 Tom Guest 5 George Robson 4 Ollie Kohn 3 Mark Lambert 2 Joe Gray 1 Joe Marler.
Replacements: 16 Chris Brooker, 17 Rob Buchanan, 18 Will Collier, 19 Tomas Vallejos, 20 Will Skinner, 21 Karl Dickson, 22 Rory Clegg, 23 Tom Casson.
Wasps: 15 Mark Van Gisbergen, 14 Richard Haughton, 13 Ben Jacobs (capt), 12 Steve Kefu, 11 Tom Varndell, 10 Riki Flutey, 9 Nic Berry, 8 Andy Powell, 7 Serge Betsen, 6 Richard Birkett, 5 James Cannon, 4 Marty Veale, 3 Zak Taulafo, 2 Tom Lindsay, 1 Tim Payne.
Replacements: 16 Michael Mayhew, 17 Charlie Beech, 18 Ben Broster, 19 Joe Launchbury, 20 Sam Jones, 21 Joe Simpson, 22 Dave Walder, 23 David Lemi.
Referee: Wayne Barnes