10 Dec, 12:23
Ireland's John Lacey will referee his first ever RBS 6 Nations match in February, while Alain Rolland is also included in the Elite Panel in what is his last season.
Two yellow cards in quick succession for Johnny O'Connor and Fetu'u Vainikolo further hampered Connacht's chances, with winger Williams soon put through for his second try.
Eric Elwood's men were down to 12 players briefly after their replacement hooker Ethienne Reynecke was sin-binned along with Joe Marler for trading punches.
Connacht showed great resilience as Andrew Browne went close to scoring a try and then the returning O'Connor broke off a lineout maul to touch down on the hour mark.
However, 'Quins replied just two minutes later as Ugo Monye scorched over to celebrate his 200th appearance for the London club.
With the bonus point secured, the hosts built for a strong finish and late tries from replacements George Lowe and Ben Botica added a slightly flattering gloss to the final scoreline.
Matthew Jarvis, deputising for the injured Dan Parks, had kicked a tenth minute penalty at the Stoop to split a brace of kicks from Nick Evans.
But Harlequins gradually built pressure through their forwards, forcing the 34th minute penalty try and another late scrum yielded three more points for Evans.
The 16-3 half-time margin did not tell the full tale of the first 40 minutes though, as Connacht enjoyed much of the early play and Tiernan O'Halloran threatened on a couple of occasions.
Elwood's charges got off to a purposeful start, their youngsters clearly revelling in the opportunity of taking on the reigning English champions and Pool 43 leaders.
New Zealander Evans opened the scoring after Connacht were pinged for not releasing, although it was the visitors who were full of running early on.
Winger O'Halloran did well to gobble up the restart and Denis Buckley and Eoin Griffin made decent incisions, setting up a well-struck penalty leveller from Jarvis.
It was a deserved three points for the westerners who led the possession and territory statistics and scrapped furiously at the breakdown to deny the hosts quick ruck ball.
However, Connacht were caught overplaying near their 22 and Evans kicked 'Quins back in front with his second penalty.
A further setback for Connacht saw the in-form Eoin McKeon withdrawn with an ankle injury, with Michael Kearney coming on and Browne switching to number 8.
Harlequins picked up the pace midway through the half with Mike McCarthy and Michael Swift spearheading a muscular defence up front from the province.
Off lineout ball, Connacht's young backs showed what they could do as David McSharry did well to draw in a couple of defenders to release O'Halloran through a midfield gap.
Disappointingly for the Connacht fans, Mike Brown got over an ensuing ruck ball to force a relieving penalty for Conor O'Shea's side but the visitors' expansive play was certainly testing the Londoners.
Evans missed a kickable penalty and another surging run from the busy O'Halloran brought Connacht downfield from a quickly taken 22.
'Quins were finding their tempo though, Danny Care getting their big ball carriers involved and they shunned a left-sided penalty kick in favour of touch.
The penalties were beginning to cost Connacht - a flurry of them in their own half - and two collapsed five-metre scrums with 'Quins forward grunt coming to the fore led to a penalty try.
Evans converted for a 13-3 advantage and after the Connacht front row were penalised at a central scrum, the out-half widen the gap to 13 points.
Connacht were caught cold on the resumption. Brown's arcing cross-field run led to him putting winger Williams away for the right corner where he scored despite O'Halloran's last-ditch challenge.
Evans swung over the touchline conversion and Connacht were now facing a mountainous task, especially after O'Connor was yellow carded for obstructing Williams off the ball.
Good broken-field running from Brown set up another try-scoring chance, Evans dangling a kick to the left corner for Monye and the covering Vainikolo was penalised for a push.
The Tongan winger saw yellow for his indiscretion and 'Quins, in the very next phase, spread the ball wide for Williams to score his second try on the right. This time Evans failed to add the extras.
With Reynecke and Marler flashed yellow cards in the 59th minute, Connacht regrouped and a quick lineout throw from Ronan Loughney put Browne charging for the try-line. Tackled, the back rower's pass back inside was covered by 'Quins.
But Connacht used another close-in lineout to secure that deserved try through O'Connor, who controlled the ball at the base of the maul and then lunged over past Monye near the line. Replacement Miah Nikora missed the conversion.
Stung into action, Harlequins bit back as the England and Lions-capped Monye broke from halfway and beat the unfortunate O'Halloran, who pulled up injured, to notch the bonus point score.
The benches were emptied in a strength-sapping final quarter and despite some excellent covering from the likes of Robbie Henshaw and O'Halloran's replacement James Loxton, the Connacht line was breached on two more occasions.
The westerners were caught flat as Evans' sidefooted kick through set up a 78th minute try for the onrushing Lowe. Botica converted and then with assistance from captain Chris Robshaw, the Kiwi rumbled over for a sixth touchdown in the final seconds.
|Harlequins Score Card|
|Connacht Rugby Score Card|