|Harlequins (13) 18|
|Tries: Alofa, Ward Cons: N Evans Pens: N Evans, Jackson|
|Leicester Tigers (3) 27|
|Tries: Harrison, Penalty try, Burns Cons: Burns 2, Williams Pens: Burns 2|
Leicester's 24 second-half points ended Harlequins' unbeaten home league record to move level with fourth-placed Bath.
After the ball bounced off the corner flag, Alofa Alofa got the only try before the break as the hosts led 13-3.
Sam Harrison dotted down for Tigers and they were given a penalty try when Adam Thompstone was taken out of the air by Aaron Morris, who was sin-binned.
Freddie Burns added a third when he intercepted to score before Dave Ward grabbed a late Quins consolation try.
Ruaridh Jackson missed the conversion from out wide with the final kick of the game, costing the West London side a losing bonus point, as the visitors recorded their first win away from Welford Road since November.
John Kingston's Quins side had won all seven of their Premiership matches at the Twickenham Stoop this season, but are now 10 points behind Leicester in sixth.
Leicester fly-half Burns, who will join Bath in the summer, scored 15 points to help Aaron Mauger's team over the line.
Tigers wing Thompstone was given a head injury assessment after being illegally felled by Morris, but he was able to continue, but Quins fly-half Nick Evans was forced off on his return from seven weeks out with a shoulder injury.
Harlequins director of rugby John Kingston:
"This was the cup final for us and by losing it, we know that we are not going to finish in the top four and the best we can do is to finish in the top six.
"We are an act in progress and I'm really clear as to where we are and what needs to be done, and I won't be stopping until I make the necessary adjustments.
"We made too many mistakes and didn't deserve to win, although I thought we deserved to get something out of it."
Tigers head coach Aaron Mauger told BBC Radio Leicester:
"I'm obviously impressed and pleased with the result, and large parts of the performance.
"It was nice and pleasing to start well, but I just don't think we were clinical enough to put points on the board and put Quins under more pressure early on.
"Five or six weeks ago we were in a bit of a hole really, and it was the belief and confidence in the group that allowed us to dig our way out of it."
Harlequins: Morris; Yarde, Marchant, Hopper, Alofa; N Evans, Mulchrone; Lambert, Buchanan, A Jones, Merrick, Matthews, Chisholm, Ward (capt), Luamanu.
Replacements: Gray, O Evans, Shields, South, Naoupu, L Jones, Jackson, Walker.
Leicester: Veainu; Thompstone, Betham, Mermoz, Pietersen; Burns, Harrison; Genge, McGuigan, Bateman, Wells, Barrow (capt), Fitzgerald, O'Connor, McCaffrey.
Replacements: Youngs, Brugnara, Betts, Williams, Hamilton, Kitto, Williams, Smith.
Referee: Dean Richards.
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