Loni Uhila was sent off as Top 14 leaders Clermont Auvergne lost 16-12 at La Rochelle and second-placed Lyon beat Racing 92 on Saturday.
Three Ihaia West penalties gave La Rochelle a 9-0 half-time lead at Stade Marcel-Deflandre, but Damian Penaud's try just after the break got Clermont on the board.
The 2016-17 champions were reeling 57 minutes in, though, when Uhila was dismissed for a high tackle on Pierre Bourgarit, reducing them to 12 men soon after Peceli Yato and Etienne Falgoux were sin-binned.
A penalty try increased the home side's lead and although John Ulugia's score 10 minutes from time gave Clermont hope, they were unable to avoid a second defeat of the season.
Lyon closed the gap to four points with a 19-13 victory at Paris La Defense Arena.
Lionel Beauxis booted 14 points and Liam Gill scored a first-half try, ensuring Simon Zebo's five-pointer in the second half was not enough for Racing.
Perpignan have now lost seven out of seven after going down 12-9 at Pau, while second-bottom Agen suffered a 10-0 defeat at Toulouse.
Grenoble celebrated only their second triumph of the campaign, Adrien Latorre, Leva Fifita and Etienne Fourcade scoring first-half tries in a 28-25 win over Bordeaux-Begles.