Marcus Smith's late penalty secured Harlequins a surprise 27-17 victory away to former two-time Champions Cup winners La Rochelle on Sunday and a top-two finish in Pool 3, while the French side were eliminated.
Earlier, Louis Bielle-Biarrey scored an opportunistic late try to complete his hat-trick and secure a 27-15 victory for Bordeaux-Begles at Bristol that earned them the top seeding for the knock-outs.
Glasgow made sure they would also enjoy home advantage all the way to the semi-finals with a dominant 28-3 victory over Saracens in Pool 1.
La Rochelle needed only a losing bonus point to book their place in the last 16 at fellow two-time winners Leicester's expense but were undone by a late Quins flourish.
"The club and myself lost big tonight, it's unacceptable," fumed La Rochelle's head coach Ronan O'Gara, a two-time Champions Cup winner with Munster as a fly-half.
"It's especially unacceptable when you see the number of opportunities we had to stay in the competition. That makes me even more frustrated."
The 2022 and 2023 winners were pushing for a clinching score in the final 10 minutes when level at 17-17, only for Quins to burst upfield and break their hearts.
Tom Lawday scored in the corner with eight minutes left and after Smith converted, La Rochelle trailed by seven points.
That would have been enough to earn the losing bonus point but Smith's last-gasp penalty denied them that and sent them into the Challenge Cup knock-outs.
La Rochelle had drawn first blood on 11 minutes with a try from wing Davit Niniashvili but Quins exploited a yellow card to French prop Uini Atonio to score through backs Cadan Murley and Tyrone Green for a 17-7 lead.
However, Quins had flyhalf Smith binned in the second half and La Rochelle took advantage to draw level through unconverted tries by former England wing Jack Nowell and scrum-half Thomas Berjon.
But Smith's return to the field proved costly for the French side, while victory ensured Quins will play at home in the last 16.
🏅 Smithy is your Player of The Match in La Rochelle!
— Harlequins 🃏 (@Harlequins) January 18, 2026
Well played, @MarcuSmith10 🙌#COYQ | #SRvHAR pic.twitter.com/rx4eTFzPf2
BIELLE-BIARREY AND TODARO TREBLES
In southwest England, hooker Gabriel Oghre put hosts Bristol on the board first but wing Bielle-Biarrey grabbed two tries and centre Nicolas Depoortere one as Bordeaux took a commanding 17-5 lead into the break.
South African centre Benhard Janse van Rensburg's try closed the gap with Sam Worsley converting and adding a penalty to bring the Bears to within five points of the reigning champions.
But Bielle-Biarrey's quick reflexes and dart of speed got him first to a loose ball to seal the win that guaranteed Bordeaux top spot in Pool 4 and the number one seeding in the knock-outs after a perfect group record of four bonus point wins out of four.
They will have home advantage through to the semifinals.
Last season's beaten finalists Northampton leapfrogged Bristol into second place in the group as Italian wing Edoardo Todaro scored a hat-trick of first-half tries in an entertaining 43-28 victory over Scarlets.
The Welsh region had taken a surprise lead through tries from Ryan Elias and Archie Hughes but once the Saints juggernaut chugged into gear, they seemed to be running away with it.
Ollie Sleightholme and Craig Wright added to Todaro's hat-trick but Scarlets bounced back through tries from Sam Costelow and Jac Davies to leave them within a converted try of the victory they needed to stay in the tournament.
But Anthony Belleau scored a penalty and replacement flanker Callum Chick added a try to eliminate Scarlets and send South Africans Bulls into the last 16.
Glasgow will be the second seeds in the last 16 after matching Bordeaux's record of four bonus-point victories.
First-half tries from Ollie Smith, Kyle Steyn and George Horne, who added three conversions, had Glasgow in the driving seat before Seb Stephen's score after the final hooter secured the bonus point.
Saracens qualified for the last 16 despite defeat but missed out on a home match in the knockouts.