Exeter Chiefs completed an Aviva Premiership double over leaders Harlequins with a stunning 27-16 upset at the Stoop yesterday.
The result, following on from the Devon side's 42-28 win at Sandy Park in October, means the champions will be replaced at the top of the table if rivals Saracens beat London Welsh at home today.
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 Exeter defence.
Conor O'Shea, Quins' director of rugby, said: "It's the second time we've been outmuscled by Exeter. I am frustrated, as every time we edged in front we conceded a penalty, but credit to Exeter as they ramped it up at the end."
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 to raise their hopes of making next season's Heineken Cup.
After Quins took a seventh-minute lead with a splendid try from Monye, Steenson struck back with three consecutive penalties to put Exeter in front. The advantage then ebbed and flowed until the final 10 minutes, when Exeter took command.
Haydn Thomas sent Welch over for the decisive try and, with six minutes left, Arscott brushed away a tackle from Ben Botica to run 35 metres for a converted score.
The Ospreys, who drew 13-13 in their RaboDirect Pro12 match at Munster last night, have announced the signing of Tito Tebaldi, the Italian scrum-half, on a two-year deal.
After a tight first half Damien Varley's try put Munster ahead, but Jonathan Thomas touched down for Ospreys and Matthew Morgan's conversion levelled the scores.
Rocky Elsom, the former Australia flanker, has joined Toulon as cover for the injured Stéphane Munoz.