Harlequins blew Pool Six wide open as they secured a stunning Heineken Cup win away to Toulouse.
Tries from Mike Brown and Joe Gray helped Quins into a 15-3 lead.
A Yoann Maestri try and Jean Marc Doussain's boot saw Toulouse hit back to lead 19-15 after an hour.
But Brown scored a second try and Nick Evans landed a conversion and penalty and although Doussain went over for Toulouse, Evans killed the game off with two further penalties.
The win moves Harlequins second in the table, but they are a point behind Toulouse thanks to the losing bonus point secured by the French giants.
It was an exhilarating encounter in south-west France and although the second half lacked some of the quality of the first, it more than made up for it in tension as Toulouse came from behind to lead before Quins somehow found a second wind and clawed their way back to win.
The one negative in the game was an appallingly soft pitch which churned up at every scrum and required running repairs throughout.
But Premiership leaders Quins will not be complaining as their incisive attack and committed defence made a big mess of Toulouse's apparently serene progress to the last eight.
Outgunned in terms of sheer power up front, Quins looked to play a high-tempo, mobile game when in possession in order to stretch the Toulouse defence out wide.
They were also not afraid to infringe at the ruck as they tried to slow down Toulouse when the hosts had the ball, and with the visiting scrum penalised at the first two engagements it meant the French side were handed four kickable penalties in the first 20 minutes.
Fortunately for the Londoners, Luke McAlister could only land one of them, and that was rapidly cancelled out by fellow ex-All Blacks player Evans for Quins.
Harlequins then took the lead with a fine try as Brown latched on to Evans's slide-rule grubber kick and Quins extended their advantage as Matt Hopper's off-load out of the back of the hand sparked a flurry of quick passes to enable Gray to power over.
Evans converted and Quins held an unlikely 15-3 lead after 24 minutes.
The Londoners were flying into every tackle but Toulouse were a threat every time they attacked, with replacement centre Florian Fritz scattering men every time he had the ball, and they scored just before half-time when Maestri was on Jean Bouilhou's shoulder to cross under the posts.
Doussain's conversion cut the gap to five points and two penalties soon after the re-start put the hosts back in front for the first time since the early stages.
It could have been even better for Les Rouges et Noirs but McAlister pulled a drop-goal just wide, William Servat was held just short and then held up over the line, while Vincent Clerc was tackled a yard short by impressive Quins full-back Brown.
Doussain also missed a penalty but the scrum-half rediscovered his accuracy on the hour mark to nudge Toulouse into a four-point lead.
A Toulouse victory seemed inevitable but replacement Quins winger Tom Williams had other plans and after he won back the re-start Brown grabbed his second try of the contest.
Evans converted and then kicked a penalty to make it 25-19 to the visitors.
Quins had Nick Easter yellow-carded two minutes later and Doussain burrowed over in the corner, but he could not land the conversion and Evans made the game safe with his fourth penalty.
Harlequins skipper Chris Robshaw:
"Last week we were in too much awe of them [Toulouse] but today we gave it everything.
"We stuck in there especially in the second half when they seemed to be gaining the momentum.
Toulouse scrum-half Jean Marc Doussain:
"We can't lose our heads, we have our destiny in our own hands. It's a pity we couldn't win, having won there last week. If we had taken our chances we would have won.
"We knew how Harlequins would play, but perhaps they raised their level compared to last weekend."
Toulouse: Medard; Clerc, David, Jauzion, Matanavou; McAlister, Doussain; Montes, Servat, Johnston, Millo-Chluski, Maestri, Bouilhou, Dustautoir, Sowerby.
Replacements: Fritz for David (24), Human for Montes (41), Botha for Servat (65), Poux for Johnston (61), Lamboley for Millo-Chluski (67), Picamoles for Sowerby (65).
Not Used: Burgess, Nyanga.
Harlequins: Brown; Stegman, Hooper, Casson, Monye; Evans, Care; Lambert, Gray, Johnston, Vallejos, Robson, Robshaw, Skinner, Easter.
Replacements: Brooker for Gray (53), Wallace for Skinner (46), Williams for Stegman (61).
Not Used: Mayhew, Sinckler, Matthews, Bolt, Urdapilleta.
Sin Bin: Easter (68).
Ref: Alain Rolland (Ireland).