Mark McCall accepts that "everything is on the line" when holders Saracens fight for European Champions Cup survival tomorrow.

Saracens, European champions in 2016 and 2017, will effectively see hat-trick hopes disappear if they fail to beat the Ospreys in Swansea.

Successive Pool Two defeats against French heavyweights Clermont Auvergne last month, which included a 46-14 drubbing at home, have left McCall's men in danger of pool-stage elimination for the first time since 2011.

Clermont can win the group and secure a last-eight place with victory over already-eliminated Northampton tomorrow, leaving Saracens chasing one of three best runners-up spots by winning their final two games against Ospreys and Saints.

"It's a play-off game, it's a big game," Saracens rugby director McCall said.

"The players want to be involved in quarter-finals, they want to be involved in those games. If we lose, we won't be. It's as simple as that really.

"It is the first time for a few years we've been in a fifth pool match where everything is on the line. It's a new experience for us even to be in that position so early in the competition. It is what it is.

"They (Ospreys) have had a good couple of months, but more than that, they've got good players right through their team that are playing well.

"Their half-backs are outstanding, they have got a great scrum, they are well coached, they are at home, so there are lots of reasons why it is going to be a difficult game."

Saracens are boosted for the Liberty Stadium clash by Wales international Liam Williams returning from a groin-related injury and making a first appearance since the autumn Tests two months ago.

Wasps and Exeter are also battling to stay afloat, with Wasps currently three points off second spot in Pool One and Aviva Premiership champions Exeter four points behind second-placed Montpellier - their Sandy Park opponents on Saturday - in Pool Three.

Wasps head to Premiership rivals Harlequins, and rugby director Dai Young said: "We definitely need two wins. We may even need nine points, which would mean we need to get a bonus point from somewhere.

"It's always a tight game against Quins, and we don't expect anything different this weekend, even though the competition is over for them.

"I think they would quite enjoy putting a dent in our ambitions as well, so we are not expecting any favours. We know it's going to be a tough game and we know we are going to have to play some good stuff."

Exeter, meanwhile, will be out of the tournament if Montpellier leave Devon with a win after the Chiefs lost home and away fixtures against group leaders Leinster last month.

"We will fight for everything until it's gone," Exeter rugby director Rob Baxter said.

"We have kept it simple to the players this week. We've just told them to turn up in the right frame of mind, lay every bit of emotion they have out on to the field and what will be, will be."

Elsewhere on Saturday, Pool One leaders La Rochelle tackle Ulster in Belfast, while Sunday's action takes Pool Four strugglers Leicester to Castres, Racing 92 host Munster in the same group, Pool Three favourites Leinster entertain Glasgow and Toulon tackle Benetton in Pool Five.