Second-half tries from Damian Welch and Luke Arscott snatched a memorable 27-16 win for Exeter Chiefs over Harlequins at The Stoop.
Quins headed into the game in very strong form, while the Chiefs were fresh from a comfortable victory over London Welsh.
The game's first try came after just six minutes, when Ugo Monye surged on to a neat short ball to touch down. Nick Evans added the conversion to make it a perfect start for the hosts.
But Quins' indiscipline cost them in the minutes that followed as Gareth Steenson struck two penalties to cut the deficit to a single point.
Steenson then added another three-pointer on the half-hour to put the Chiefs in front for the first time, though Evans' effort less than five minutes later restored a narrow advantage for Harlequins.
The two teams traded further penalties following the restart, with Steenson's brace for Exeter cancelled out by two three-pointers from Quins substitute Ben Botica.
A great break initiated by Steenson then saw the Chiefs go close, before the pressure finally told as Damien Welch crashed over from short range after the ball was worked right.
The concession prompted a frenzied response from the hosts and they appeared capable of battling their way back in front, but a stunning second try from the visitors left Quins with a mountain to climb, as Luke Arscott broke through some weak defence before swan-diving over under the posts.
The excellent Steenson added the extras to make it 16-27, and, though Quins rallied in search of a losing bonus point, the Exter defence held firm to scupper any hope of consolation.
Pens: Evans, Botica 2
Tries: Welch, Arscott
Pens: Steenson 5
Harlequins: 15 Ollie Lindsay-Hague, 14 Tom Williams, 13 Matt Hopper, 12 Jordan Turner-Hall, 11 Ugo Monye, 10 Nick Evans, 9 Karl Dickson, 8 Tom Guest, 7 Luke Wallace, 6 Maurie Fa'asavalu, 5 George Robson, 4 Nick Easter (C), 3 James Johnston, 2 Joe Gray, 1 Mark Lambert.
Replacements: 16 Rob Buchanan, 17 Darryl Marfo, 18 Will Collier, 19 Charlie Matthews, 20 Joe Trayfoot, 21 Jordan Burns, 22 Ben Botica, 23 George Lowe.
Exeter Chiefs: 15 Luke Arscott, 14 Jack Nowell, 13 Sireli Naqelevuki, 12 Phil Dollman, 11 Matt Jess, 10 Gareth Steenson, 9 Haydn Thomas, 8 Richard Baxter, 7 James Scaysbrook, 6 Ben White, 5 Damian Welch, 4 Tom Hayes, 3 Alex Brown, 2 Jack Yeandle, 1 Carl Rimmer.
Replacements: 16 Neil Clark, 17 Ben Moon, 18 Craig Mitchell, 19 James Hanks, 20 Dave Ewers, 21 Will Chudley, 22 Ignacio Mieres, 23 Ian Whitten.
Referee: David Rose
Assistant referees: Roy Maybank, Andrew Pearce
Television match official: Graham Hughes