Harlequins director of rugby Conor O'Shea was shaken by his side's poor performance as the league leaders lost 27-16 to Exeter on Saturday.
Quins were marginally the better side for much of the contest but they could only manage one try, from Ugo Monye, and lost 27-16 at The Stoop.
The hosts led with 12 minutes remaining but Exeter, 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.
Exeter beat Quins 42-28 at Sandy Park in October, and the latest defeat means Saracens go to the summit with victory over London Welsh on Sunday.
"That's the second time we've been outmuscled by Exeter," said O'Shea. "There was never any massive intensity or energy in our play as they held on to the ball and won all the collisions.
"I'm frustrated as every time we edged in front we conceded a penalty, but all credit to Exeter as they kept coming and coming and ramped it up at the end. We were tired out with all the tackling we had to do as a result of coughing up possession."
Quins have difficult league tests away to both Saracens and Gloucester ahead of them but O'Shea was still not entirely downbeat.
He said: "It was not a train crash today; it's a long haul but we will have to rectify this performance as we face two tough games now but it's all about finishing in the top four and that is in our own hands. When we lost at Exeter in October we went on a winning run of 10 games so we learn our lessons well."
Exeter coach Rob Baxter was delighted with the win.
He said: "We showed great intensity and a great work ethic especially in our defence as they are the top try scorers in the league. Today our performance was spurred by ambition not fear as we are safe from relegation."
With five games to go, Baxter still holds hopes of securing another Heineken Cup spot.
He said: "We saw today as the beginning of a cup run. We've taken nine points from the last two games and you soon move up the table. It was good to play on a firm pitch which suits us but I'm still frustrated by our loss at Sale as we should have finished that game off."
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