Stade Francais defeated Paris rivals Racing 92 by 27-23 on Sunday to keep their Top 14 play-off hopes alive in a bruising match which saw both sides have a man sent off in the first half.
In the first clash between the sides since a controversial merger fell through, Stade ran in three tries to Racing's two but a penalty by Dan Carter 10 minutes from time allowed the visitors to take home a losing bonus point.
Defending champions Racing remain in the sixth and final play-off spot with 58 points, just ahead of Stade, also with 58, with one weekend of the regular season left.
"Our chances of qualifying are tiny but they do exist," said Stade coach Gonzalo Quesada.
"We will go to Montpellier next Saturday without some players who will be rested (ahead of the European Challenge Cup final against Gloucester) but we will still have a very competitive team."
The two sides were locked at 13-13 at the break with a try apiece from Stade skipper Sergio Parisse on 34 minutes and veteran Racing centre Casey Laulala on the stroke of half-time.
Three minutes before the interval, both packs squared up and locks Wenceslas Lauret of Racing and Stade's Pascal Pape were red-carded.
Pape, 36, in what was probably his last match at the Stade Jean-Bouin after 10 years in the first team, received a standing ovation from the bench as he walked off.
Stade stormed into a 27-13 lead after two tries in the space of four minutes early in the second half from winger Djibril Camara and centre Jonathan Danty, who outsprinted All Blacks winger Joe Rokocoko to touch down his own kick ahead.
Racing prop Ben Tameifuna cut the deficit with a 62nd minute try which Carter converted.
Carter then added the crucial late penalty to complete his 13-point haul.
Double World Cup winner Victor Vito crossed for a brace of tries as leaders La Rochelle withstood a fighting comeback from Montpellier to guarantee they finish first in the regulation season.
Montpellier's giant Fijian winger Nemani Nadolo had a try ruled out by the television match official in extra-time in a nail-biting match which sees La Rochelle now on 84 points.
"We won, which was our goal," said La Rochelle backrow forward Romain Sazy.
"We went missing for the final 10 minutes and they scored tries willy-nilly, we respected nothing."
A losing bonus point for Montpellier sees the southern club in third on 72 points, two behind Clermont, who occupy the second automatic semi-final spot. Toulon lie fourth (65), ahead of Castres (63).
Willem du Plessis, with three South African namesakes also on Montpellier's team sheet, opened the scoring when he crashed over for a try he also converted.
La Rochelle's Australian fly-half Brock James and Du Plessis exchanged penalties shortly after in what would be a fine kicking display in rainy conditions by the two players.
La Rochelle looked to have wrapped the game up with tries from Vito, hooker Jeremie Maurouard and Pierre Aguillon.
But at 40-16 up after 64 minutes, they hadn't counted on a ferocious comeback from the visitors, who scored three tries (penalty, Nadolo and Duhan van der Merwe) in the final six minutes after the sin binning of James for pulling down a maul.