The home side took an early lead when lock George Skivington crossed in the second minute, but Saracens went in at the break only 11-8 behind thanks to winger Paul Bailey's 11th-minute try.
Wasps struggled to impose themselves in the second half against a youthful Saracens outfit until Paul Sackey broke loose on the right to charge straight through an unsuspecting defence in the 66th minute.
And from then on Wasps showed their class, running in three more tries through Josh Lewsey, Sackey and Eoin Reddan after the visitors' number eight Steffon Armitage was sin-binned in the 70th minute.
Gavin Henson's comeback was put on hold as Ospreys ran in five tries, two of them from Adrian Cashmore, in a 43-28 victory at Bristol.
Wales centre Henson, who has not played since being concussed while playing for the British & Irish Lions against the All Blacks in July, failed a late fitness test after suffering back spasms.
But Ospreys, who had already lost to Bath and Gloucester, were strong enough to see off Bristol's challenge despite a despite a hat-trick of tries from Marko Stanojevic for the home side.
There was nothing riding on the result of this game - Bath having already booked a semi-finals spot with their win over Gloucester - so both sides rested many senior players.
Damian Karauna, Steven Tandy and James Bater were Ospreys' other try-scorers as they pulled away in the second half after Bristol had lead 28-25.
A brace of tries from Paul Gustard and an impressive performance from young fly-half Ross Laidlaw helped London Irish to a 27-23 win over Cardiff Blues which ended the visitors' Powergen Cup hopes.
The result was nothing more to the Exiles than a run-out for the up-and-coming players as they were already out of semi-final reckoning following earlier defeats to Saracens and Wasps.
Cardiff still had a slim chance to qualify for the Millennium Stadium semi-final showdown in March if Wasps also suffered a bad day against Saracens.
That scenario did not materialise as the Blues failed to mount any real attacking form until late in the game at the Madejski Stadium.
Gustard's double and Rodd Penney's five-pointer gave Irish a 21-6 lead before half-time, but Mike Phillips and Gareth Williams scored tries either side of the interval to reduce the Exiles' lead to just a point.
Barry Everitt and Jamie Robinson traded penalties before Shane Geraghty's 89th-minute drop goal sealed victory for the home side.