Exeter pulled off a stunning Aviva Premiership upset as leaders Harlequins were sent crashing to defeat at the Stoop.
Quins were marginally the better side for much of the contest but aside from an Ugo Monye try that put them in front early on, they could not break down a stubborn visiting defence.
The hosts led with 12 minutes remaining but the Chiefs, inspired by fly-half Gareth Steenson, produced two late tries from Damian Welch and Luke Arscott as their bid to qualify for next season's Heineken Cup was given a boost.
The result means Exeter have done the double over the Quins after they also defeated the champions 42-28 at Sandy Park in October, while Harlequins' slip means Saracens can replace them at the summit with victory over London Welsh tomorrow.
Exeter were without Dean Mumm, Hoani Tui and Jason Shoemark who were all injured in last week's emphatic win over London Welsh, with Tui's absence allowing Alex Brown to make a first premiership start at prop.
Nick Evans returned for the Quins after a three-week absence with an ankle injury while captain Nick Easter appeared at lock for the second time this season with Tom Guest at eight.
Harlequins took a seventh-minute lead with a splendid try.
After winning a line-out in the Exeter 22, Jordan Turner-Hall's well-timed allowed Monye to break the defensive line at speed and race away for the try which Evans converted.
Steenson put the Chiefs on the scoreboard with two penalties, awarded after Harlequins first offended at a scrum then had Karl Dickson ruled offside.
The hosts suffered a blow when young full-back Ollie Lindsay-Hague was tackled heavily and limped from the field to be replaced by Ben Botica, with Evans switching to full-back.
Evans missed a penalty for Quins before a third successful kick from Steenson put Exeter ahead.
Quins continued to dominate possession and territory and Evans' penalty just before the interval handed them a deserved 10-9 half-time lead.
Within four minutes of the restart Exeter were back in front, Steenson was again on target, but this Quins hit back with two penalties from Botica, who had taken over the kicking duties from Evans.
Referee David Rose continued to blow for transgressions as the flow of the game suffered, and from his next penalty award Steenson kicked through the posts for the visitors.
With Quins ahead 16-15 as the final 10 minutes approached, Exeter produced their best move of the game.
Showing great skill, Steenson evaded three players on halfway to race clear but with a try appearing to be a certainty Arscott wastefully knocked-on with the line at his mercy.
The Chiefs then had their best spell of the match as they camped inside the Quins 22.
Steenson looked a certain scorer until he was ankle-tapped but they kept up the pressure for Haydn Thomas to send Welch over for the try which Steenson was unable to convert.
It was now all Exeter and with six minutes remaining Arscott brushed away a weak tackle from Botica to run 35 metres for a converted try that sealed a memorable victory.