Wasps collapsed to a dramatic 33-28 defeat by Harlequins that all but ends their European Champions Cup quest and hands Ulster a boost ahead of next week's trip to Coventry.
Les Kiss's men are top of Pool 1 courtesy of a 20-13 win over La Rochelle, who host out-of-contention Quins next week.
Mathematically, Wasps can still qualify for the knock-out phase, but would need a four-try win with an 11-point margin to be in contention for second place (assuming the French side pick up a single point against the Londoners).
That equation also requires Ulster not to pick up a scoring bonus point.
La Rochelle will be favourites to top the table and a five-point haul would mean Ulster would need to pick up five points at Wasps to maintain their place.
A runners-up quarter-final place would then depend on how the other pools play out.
Jamie Haskell flattened Jamie Roberts with a shoulder-high challenge in the 76th minute that was also late, receiving a straight red card from referee Roman Poite and ending the groggy Wales centre's afternoon.
The England flanker faces a suspension that could affect his availability for the Siz Nations, but his more immediate concern will be Wasps' implosion at Twickenham Stoop.
Leading 21-0 after 30 minutes, they leaked a succession of tries, grabbed the bonus point, before falling apart once again as touch downs from Danny Care, Elia Elia and James Chisholm completed a remarkable fightback by Quins.
Whats done is done, but just want to publicly apologise to @Jamiehuwroberts. Never my intention to hurt another player. Got my height wrong, paid the price, which happens. You know me, never one to do anything subtly. Credit to @Harlequins and apologies to @WaspsRugby fans— James Haskell (@jameshaskell) January 13, 2018
Harlequins' interest in Europe was extinguished long ago - they had lost all four previous matches of this season's group phase - but a team weakened by injury and suspension fought to the end and were rewarded.
It was the 80th minute when man of the match Chisholm picked and drove over from close range as shell-shocked Wasps rubbed their eyes in disbelief at having thrown the game away.
An uneventful first quarter came alive in the 17th minute when Wasps attacked from a line-out and were rewarded by crossing several phases later.
Drives from Haskell and Jake Cooper-Woolley sent Quins backwards before quick hands by Willie le Roux and Thomas Young enabled Kyle Eastmond to accelerate over the whitewash.
Wasps struck for a second time three minutes later when Care and the superb James Lang spilt the ball between them, enabling a counter-attack that saw Nizaam Carr run 60 metres to cross.
🗣 "We worked so hard to get something out of it, it's about momentum and winning as many games as we can"— Harlequins 🃏 (@Harlequins) January 13, 2018
📽 Man of the Match James Chisholm following the win over Wasps pic.twitter.com/RjuvXiyaML
Both tries had been scored with Quins prop Lewis Boyce in the sin-bin for tackling Ashley Johnson without the ball and it was the Wasps hooker who touched down for the third.
Route one was the path over the line this time, Joe Launchbury catching the line-out ball as the prelude to a drive that ended when Johnson grounded the ball.
Quins took advantage of a broken field to begin their response, Lang punting ahead for Ross Chisholm to win the foot race and score.
The match was now finely poised and when Care set-off from a five-metre scrum, delaying his pass for Lang to skip over, the home side were over for a second.
Quins' fightback was interrupted 14 minutes after half-time and once again a line-out provided the platform for the undoing.
Young carried into the heart of the defence and with gaps appearing out wide Danny Cipriani was able to send former Leinster man Brendan Macken through a gap to seal the bonus point.
Christian Wade performed a try-saving tackle on Tim Visser but the pressure continued to build through a succession of short-range drives that were completed when Care burrowed over.
All the momentum was with Harlequins and with replacement fly-half Marcus Smith orchestrating play brilliantly, they claimed their fourth try that started with Visser's break inside and finished when Smith sent Elia over with a long floated pass.
And Wasps' fate was sealed when James Chisholm drove across the whitewash with seconds to spare.
Billy Vunipola provided England with an injury scare as holders Saracens' European Champions Cup quarter-final hopes were left hanging by a thread.
The Saracens and England number eight went off at half-time during Saracens' 15-15 draw with the Ospreys at Liberty Stadium.
He appeared to suffer a wrist injury in only his second comeback game following three months' sidelined because of knee trouble.
Saracens, though, now realistically need a bonus point victory over Pool Two opponents Northampton next weekend to have any chance of progressing as a best runner-up after Owen Farrell's five penalties were cancelled out by five from Ospreys playmaker Dan Biggar.
Saracens' European title hat-trick bid will now hinge on results elsewhere even if they claim a five-point maximum against Saints, while Ospreys must beat Clermont in France to have any chance of progressing.
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