La Rochelle prop Joel Sclavi scored an 81st-minute try as the holders beat Ulster 7-3 on Saturday to book their place in the Champions Cup last 16.
Argentina international Sclavi bundled over in torrential rain on the Atlantic coast after Nathan Doak kicked the visitors' only points just after the hour mark.
"It was not pretty, one for the purists," La Rochelle forwards coach Donnacha Ryan told BT Sport.
"The last three minutes, there was fantastic character from the boys, a massive sense of belief and we're glad we won in the end."
The Irish side host Sale next weekend, with both teams needing a win to advance from the pool stage.
The game remained scoreless at half-time after La Rochelle fly-half Antoine Hastoy and visiting scrum-half Doak missed shots at goal.
Ulster winger Rob Lyttle also had a 36th-minute try ruled out by the television match official.
Doak opened the scoring, despite the heavy downpour, with 25 minutes left as the 1999 champions edged towards a surprise win against opponents who last lost at home in the competition in November 2019.
France No. 8 Gregory Alldritt also had a score disallowed with 11 minutes remaining as the French side, coached by Ronan O'Gara, went looking for the winning try, turning down chances to draw with a penalty.
Their efforts were rewarded after the clock passed 80 minutes as Sclavi opened his account for the season from a pick-and-go from short range.
- Meafou for France? -
Earlier, Australian-born lock Emmanuel Meafou said he hoped to be named in France's Six Nations squad after scoring to help five-time winners Toulouse beat Sale 27-5 to secure their spot in the knockout phase.
The France squad will be named on Tuesday but the 2.03m, 140kg second-row is unable to play for his adopted country before receiving citizenship, which he will reportedly obtain in time for the third-round fixture against Scotland on February 26.
"Our boys fought hard, it's good to get the win here," Meafou told BT Sport.
"Every week I try and go and play my best game and if the international call-up comes then hopefully I'll be ready for it, I'm just focusing on playing well for Toulouse now."
Four-time winners Leinster also reached the knock-out stage as they hammered Gloucester 49-14.
Durban's Sharks became the first South African side to reach the Champions Cup last 16 as they eased past Bordeaux-Begles 32-3.
Later Saturday, Elliot Daly scored a hat-trick in seven first-half minutes as three-time champions Saracens, without suspended England captain Owen Farrell, defeated Lyon 48-28 to progress.
Leicester Tigers were the first team into the next stage after the English club beat Clermont 44-29 on Friday.
Ospreys are also in contention for the knock-out rounds after a 35-29 win over Montpellier despite being outscored five tries to four.
On Sunday, three-time runners-up Racing 92 play Harlequins.