Saracens booked their second straight Heineken Cup semi-final this evening, but were made to battle tooth and nail by a 14-man Ulster in an astonishing clash which hung in the balance right to the end at the newly constructed Ravenhill.
The hosts were forced to play most of the game without Jared Payne, who was controversially sent off early in the first half after a shuddering tangle with Alex Goode which forced the Saracens full-back off the field.
Despite being down a man, Ulster refused to buckle and led 9-5 at the end of the first half but, after the home side lost key players Rory Best and Ruan Pienaar to injury, Sarries eventually got in front and held firm despite Ulster's storming finish which saw them narrow the visitors' lead to two points at 17-15.
A try in either half from Chris Ashton, along with one from second row Mouritz Botha, proved enough for Mark McCall's men with all of Ulster's points coming from the boots of Pienaar and Paddy Jackson.
The visitors had the first scoring opportunity when Rory Best was adjudged to have gone off his feet in helping to bring down Billy Vunipola, but Owen Farrell's second-minute effort sailed wide of the left upright.
The game was then held up for several minutes when Payne hit Goode in the air after following up a Paddy Jackson high kick which saw the Sarries full-back worryingly crash over Payne's shoulder before hitting the turf.
After nearly seven minutes of a hold-up which saw Goode carried off on a stretcher and the TMO consulted, referee Jerome Garces showed Payne a red card.
Ulster then lost hooker Best five minutes later with what appeared to be a foot injury yet, somehow, the home side scored first with a long-range penalty from Pienaar getting the scoreboard moving on 15 minutes.
Three minutes later the South African had another chance from a similar range after Billy Vunipola was penalised at a lineout, but this time Pienaar was wide.
Ulster then found themselves in a potentially dangerous position, but Nick Williams ignored men outside him and Pienaar knocked on from the recycle.
Then, with their first attack of substance, Chris Ashton was put away with an angled pass from Charlie Hodgson standing at out-half and the winger went in and outside auxiliary winger Luke Marshall to perform his trademark dive at the right corner.
Having failed to narrow the angle for Farrell, the conversion sailed wide but the visitors now led 5-3. Ulster needed to score next and, after Saracens butchered a scoring chance on their left wing, Tommy Bowe hacked the ball downfield and from the resulting lineout a huge maul from Ulster won a penalty on Saracens' 22.
Pienaar nailed it and Ulster were ahead again, 6-5, on the half-hour mark. Ulster then survived another close shave when Saracens lost control of the ball near the hosts' line and, with three minutes left of the half, Pienaar's long-range penalty attempt - after Mouritz Botha had obstructed Bowe - was wide of the mark.
Still, he made no mistake right at the death, out on the left, when Mako Vunipola was penalised at a scrum and Ulster trooped off 9-5 to the good.
Saracens, though, got the first score of the half with a sweeping move which was started by Billy Vunipola and was moved right, left and right again when Richard Wigglesworth picked out Botha and he touched down on the right.
Farrell again missed the extras and Ulster were clinging onto their lead at 10-9 after 48 minutes and then had to deal with losing Pienaar, who finally succumbed to the shoulder injury he had carried into the game.
Still, Ulster had a great chance to stretch their lead after Billy Vunipola was pinged for a high tackle, but Jackson's 53rd-minute strike hit the upright and stayed out.
Just before the hour, though, Saracens seemed certain to score only for Ashton and Chris Wykes to be held up by some heroic defending - which saw Andrew Trimble depart after making a heavy challenge on Schalk Brits - with the TMO denying Ashton.
The TMO was back in action again three minutes later and this time Ashton got the score following Farrell's precise cross-kick which the Sarries out-half then converted - his first success of the day - to put the visitors 17-9 in front.
Botha was then caught offside at a high kick and Ulster again got back into the game via Jackson's 69th-minute penalty and four minutes later he landed another to narrow Sarries' lead to two points.
And still Ulster battled on, but their huge attack right at the death fell short and Sarries got through to secure a Twickenham date with Clermont.