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.
Quins landed themselves in trouble at the start by losing possession and Evans was tackled behind his own line as he cleared up the mess. 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 as 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, while Goode restored Worcester's eight-point lead with his third penalty.
Euan Murray made his Warriors debut when he replaced the injured John Andress in the 35th minute. Murray's first scrum was forced into retreat by the home pack before Evans capitalised expertly by finding 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.
Evans extended the lead to five points with his fourth penalty. Worcester drew level with 12 minutes left when Classens went over in the left corner but Goode was narrowly wide with the conversion. But Evans had the final say with his 73rd-minute drop goal as Quins played the advantage after Worcester offended at the breakdown.
Northampton scored three tries in a superb first-half display as they comfortably defeated Sale 27-16 to move up to second in the Aviva Premiership.
Touchdowns from Jamie Elliott, Soane Tonga'uiha and Tom May saw them race into a 27-3 lead at the interval, with the rest of their points coming via Ryan Lamb's boot.
The Sharks' only score before the break came through Nick Macleod's penalty and, although they hit back through Rob Miller and Tommy Taylor tries, the Saints held out for a convincing victory.
The match was Sale's first since New Zealander John Mitchell took charge of first-team affairs earlier in the week. For Northampton, England full-back Ben Foden was making his first appearance since September and he was immediately into the action, running the ball back from an over-hit Dwayne Peel box-kick.
That allowed the visitors to build an early platform and, after a dominant driving maul had been illegally disrupted by the Sharks, Lamb slotted the resulting penalty. It was all Northampton in the opening exchanges but a bone-crunching tackle by Sam Tuitupou on Dominic Waldouck gave the hosts a much-needed foothold in the match.
And Mitchell's men were the next to threaten when Richie Gray's run and offload sent Cameron Shepherd through. Although they could not force their way over the line, it was called back for offside and Macleod duly converted the three-point opportunity.
However, Saints regained the lead when good work on the short side sent Elliott away on the left. The wing then showed his immense pace to touch down in the corner, with Lamb adding the extras.
Sale were struggling to deal with the sheer power of the away side's pack and a turnover at the breakdown saw them go in for another converted try when prop Tonga'uiha raced away. Lamb then kicked a drop-goal to extend the advantage to 17 points.
May rounded off an outstanding half for the play-off contenders when he latched on to a stray pass from Macleod to cross the whitewash. The Sharks came out revitalised after the interval and reduced the arrears almost immediately when Mark Cueto sent Miller clear.
Their stand-off failed to convert but he added a three-pointer to eat into that deficit further on 50 minutes. However, despite some nervy moments and Taylor gaining a consolation score for Sale, Northampton saw out the win to keep the hosts rooted to the foot of the table.
- Sports & Recreation
- Andy Goode