Nick Evans turned in a man-of-the-match performance to help Harlequins recover from 14-3 down to beat Bristol Rugby on their return to Aviva Premiership Rugby at the London Double Header.
Tom Varndell had opened the scoring for Bristol with a superb interception for his 86th Aviva Premiership Rugby try and though Evans pegged them back with a penalty, Tusi Pisi broke away for the visitors' second converted try.
Harlequins fought back, Marland Yarde scoring from a driving maul before youngster Joe Marchant danced through three tackles to go under the posts for a converted try just before the break.
Ryan Edwards struck just after half-time, but Evans kicked two more penalties to seal John Kingston's first Aviva Premiership Rugby win as director of rugby in front of 47,029 fans at Twickenham.
When Danny Care looked to attack from deep with 20 minutes gone, Varndell quickly read the England scrum-half's pass and intercepted at full-pace, galloping under the posts.
Gavin Henson converted but Evans drew Harlequins within five points when he knocked over an easy penalty with 15 minutes left in the half.
It was Andy Robinson's newly-promoted team who scored next when a loose kick found scrum-half Will Cliff, his neat sidestep taking him past Ross Chisholm, he then delivered a flat pass to Pisi who slipped through two tackles and under the posts.
Henson converted but was forced off the field through injury.
Harlequins were rewarded for their bravery when Evans turned down a kickable penalty to go for the corner with a minute to go in the first-half.
Quins claimed the lineout and drove for the whitewash and as Jamie Roberts and Yarde leant their weight to the maul, it was the winger who managed to touch down before Evans dragged his conversion wide.
A Jordan Crane penalty gifted possession back to Quins straight from the restart and Marchant made Robinson's side pay.
The 20-year-old showed great composure in heavy traffic, stepping his way through three tackles to finish next to the post, Evans converted for an unlikely 15-14 half-time lead.
Bristol surged back after the break, and when Marchant struggled to deal with Edwards' kick ahead - the youngster fumbling under intense pressure - the winger was on hand to touch down.
Pisi failed to convert from the touchline and Evans' penalty pulled Quins within a score with 20 minutes remaining before a second five minutes later gave them a lead they would not relent.