Fly-half Rory Clegg was Harlequins' stand-in hero as his kicking and creativity helped Quins beat Worcester.
Clegg filled in for Nick Evans after the New Zealander failed a pre-match fitness test and combined with George Lowe to set up Mike Brown's try.
Miles Benjamin went over in the corner to level the scores and Lowe had another try ruled out for knocking-on.
Two Matthew Jones penalties twice put Worcester ahead but Clegg matched both and landed the winning kick late on.
Victory lifted Quins one place to seventh, while Worcester - despite gaining a bonus point in defeat - remain precariously poised, two points above Sale and Leeds at the bottom.
Harlequins head coach John Kingston:
"We could have scored three tries in the first half and this win now takes us to within two points of sixth.
"There was tension in the game as it was so tight towards the end, but that's not the first time Rory has kicked a big goal for us like that.
"The lad is here to develop, to learn and possibly play international rugby."
Worcester rugby director Mike Ruddock:
"We played some good rugby out there and led twice. But once again penalties cost us dear.
"That has been the story of our season, and if we can eliminate these penalties, our game is otherwise in very good order.
"It is frustrating because, for example, the one we conceded in front of our posts just after taking the lead was avoidable.
"Our recent run has been difficult because we've had three out of four games away. But we have the necessary to climb the table."
Harlequins: Brown, Strettle, Lowe, Turner-Hall, Williams, Clegg, Dickson, Jones, Cairns, Andress, Percival, Robson, Robshaw, Skinner, Guest. Replacements: Fuga for Cairns (51). Not used: Croall, Johnston, Stevenson, York, So'oialo, Mordt, Smith.
Worcester: Latham, Pennell, Rasmussen, Tuitupou, Benjamin, Jones, Arr, Mullan, Fortey, Taumoepeau, Rawlinson, Gillies, Wood, Sanderson, Horstmann. Replacements: A Black for Mullan (47), Lutui for Fortey (47), Kitchener for Rawlinson (57), Grove for Rasmussen (66), Cracknell for Wood (70), Sourgens for Taumoepeau (80). Not Used: Powell, MacRae.
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