Clermont Auvergne racked up a club-record ninth straight win as their victory at Pau saw them pull clear at the summit of Top 14, while Stade Francais battered Oyonnax to all but secure survival and relegate their opponents.
Just two points separated the league leaders and Montpellier before Saturday's matches, but that gap was stretched to six as Clermont set a new benchmark with a hard-fought 16-10 triumph at Stade du Hameau, while Montpellier went down 36-10 at La Rochelle.
Nick Abendanon and Noa Nakaitaci's unconverted tries inside 12 minutes put Clermont in control.
Colin Slade's boot put Pau on the board shortly before half-time and, although Morgan Parra's penalty re-established a 10-point lead, the hosts were right back in it when Watisoni Votu crossed in the 52nd minute.
Another Parra penalty extended Clermont's gap to six points, though, and the visitors held firm in a nervy ending.
Montpellier had won eight straight matches themselves ahead of the trip to Stade Rochelais, but their preparations for the European Challenge Cup final against Harlequins on Friday were dented by the heavy defeat.
Zeno Kieft and Ricky Januarie touched down for La Rochelle in the first half, with Mickael Ivaldi's try and five points from the boot of Benoit Paillaugue keeping Montpellier in touch.
However, second-half tries from Leandro Cedaro and Gabriel Lacroix, plus nine points from Zack Holmes, saw La Rochelle pull away.
Stade Francais were champions last season, but a disappointing campaign this time around has left them fearing for their top-flight safety.
However, Waisea Nayacalevu and Hugh Pyle scored two apiece in a 10-try 69-8 romp over Oyonnax, a result that condemns the visitors to relegation.
Racing 92 could have closed the gap on Montpellier, but they slipped to a 33-27 loss at Brive and stay third.