Nick Evans struck the match-winning drop goal seven minutes from time as Premiership leaders Harlequins beat Worcester 22-19 at The Stoop.
The former All Blacks fly-half, who finished with 17 points, proved the difference between two well-matched sides.
He set up Quins' only try for centre Matt Hopper, converted it and also landed four penalties and the crucial drop-kick.
Winger David Lemi's second-minute try put the Warriors ahead and a second from replacement Errie Claassens put them level at 19-19 after they had two players sin-binned.
Fly-half Andy Goode's three penalties provided their other nine points as they claimed a deserved losing bonus point.
Leading try scorer Lemi marked his return from international duty with Samoa by striking first.
Quins landed themselves in trouble by losing possession as they tried to run their way out of defence and Evans was tackled behind his own line as he cleared up the mess.
Maintaining the pressure from the resulting five-metre scrum the Warriors worked the ball from left to right and back again to send Lemi over in the corner.
Goode's conversion rebounded from the post but, five minutes later, he made amends with a 40-metre penalty.
Harlequins began to take the game to the visitors and Williams raced on to a superbly-weighted grubber kick from Evans before being halted by the Worcester cover defence.
Then scrum-half Karl Dickson was held up over the line after jinking through but referee Luke Pearce, who had played the advantage, brought play back for a penalty kicked by Evans.
Within two minutes, it was cancelled out by Goode's second.
Worcester were reduced to 14 men in the 24th minute when Lemi was yellow-carded for killing the ball in front of his own line.
Quins went for touch rather than goal with the penalty but, when they were awarded a second in quick succession - from an easier angle - Evans put it over via the post.
Goode restored Worcester's eight-point lead with his third penalty and they continued to keep Quins out, stopping both Jordan Turner-Hall and Seb Stegmann just short of the line.
Scotland prop Euan Murray - signed from Newcastle following a spell on loan with French club Agen - made his Warriors debut when he replaced injured former Quins front row man John Andress in the 35th minute.
Murray's first scrum was forced into retreat by the home pack before Evans capitalised expertly by choosing the correct split second to find Hopper on an unstoppable straight run-in for his side's first try.
Evans converted and then kicked Harlequins into the lead for the first time with a penalty on the stroke of half-time after No 8 Semisi Taulava became the second Worcester player to be sin-binned as his side defended desperately.
Despite being a man down for the second time, Worcester had the better of the exchanges until Taulava returned 10 minutes into the second half.
But Quins survived the spell of pressure before Evans extended their lead to five points with his fourth penalty.
Worcester drew level with 12 minutes left when centre Alex Grove made the break to send Classens, who had replaced scrum-half Paul Hodgson only four minutes earlier, racing over in the left corner.
Goode was narrowly wide with the difficult conversion.
But Evans had the final say, with his 73rd-minute drop goal as Quins played the advantage after Worcester, who had shipped a huge amount of penalties, offended at the breakdown.
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