Former France full-back Anthony Bouthier kicked an 80th-minute conversion as Vannes claimed their first French Top 14 win of the season by edging Clermont 27-26 on Saturday.
Bouthier added the extras after Argentina back-rower Francisco Gorrissen snuck over for the Bretons, who were promoted from the second division in June.
They had trailed by 17 points with 25 minutes to play before tries from hooker Mills Sanerivi and back-rower Ioane Iashagashvili set up the late drama.
However, Vannes still remain bottom of the table, six points below Racing 92 in the relegation play-off spot.
Ex-New Zealand fly-half Harry Plummer slotted 16 points for the visitors, who have just two wins this campaign and sit in 12th place.
"Six straight defeats is tough," 34-year-old Bouthier told Canal Plus.
"You have doubts. Our real story in the Top 14 starts today."
In difficult conditions, with a yellow weather warning for wind issued in Vannes, Clermont raced to a 16-6 lead as scrum-half Baptiste Jauneau crossed and the league's leading scorer Plummer provided the remainder of their points from the tee.
They extended their lead to 23-6 after the break as prop Giorgi Dzmanashvili touched down before the Pro D2 champions launched their comeback.
Samoa's Sanerivi crossed with 25 minutes to play and Georgia's Iashagashvili then went over to set up a tense final quarter of an hour, before Gorrissen handed Bouthier the chance to end their drought.
Elsewhere, champions Toulouse extended their lead at the top to six points with a 38-33 win over Perpignan despite a spirited last half an hour from the visitors.
The Catalans' uncapped winger Theo Forner was replaced in the second half just as a precautionary measure despite suffering an ankle injury, days after being named in France's squad for next month's Nations Championship campaign.
Elsewhere, in-form Fiji flier Jiuta Wainiqolo scored his sixth try of the campaign as Lyon overcame Bayonne 31-27, two weeks out from facing France for his country in their Nations Championship opener.
"We know he has crazy X-factor," Lyon captain Antoine Deliance told reporters. "He can make the difference at any time."
Struggling Racing lost 20-17 to Montpellier in their home game moved to Le Mans, and Castres beat Stade Francais 16-8.
Later, second-placed Pau missed the chance to close the gap on Toulouse, losing 41-20 at Toulon.
Bouthier's fellow former France full-back Melvyn Jaminet scored 26 points, including the first hat-trick of his career, for the hosts, who secured their third success of the campaign.
Two other members of France's squad suffered injuries as Pau centre Fabien Brau-Boirie left the field with a knee issue and Toulon back-rower Charles Ollivon sustained a wrist problem, threatening to add to Les Bleus' head coach Fabien Galthie's already lengthy list of absentees.
On Sunday, third-placed Bordeaux-Begles and their host of France stars, including winger Louis Bielle-Biarrey, head up the Atlantic coast to La Rochelle.
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